I don't think I need a 9.9hp for a Mac25

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Maybe I just don’t understand.. but why again in this thread is current an issue. On these relatively light 25-26 foot boats, between 6 and 10 hp with the highest pitch to grittiest lowest pitch props, the boat still gets to the theoretical hull speed range so the peak speed you achieve might vary by 1 mph.

That means if you go out against a 6 knot current, with the 6 hp and a peak speed of 5.5 knots and the worst prop or the 10 hp with the best prop and a peak speed of 6.5 knots,...
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.... you should really learn how to read tide tables.. wait until the current is lower. Really, does the less than 1 mph difference in peak wot speed give you any sort of meaningful and adequate margin? If you want the hp to really matter, you should really be looking at 50 hp.

If you want the "higher thrust prop", this mostly ends up being about having a great reverse. It’s not just the higher rpm's from a lower pitch. It’s also about blade shape and how the exhaust is handled in reverse.
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
Walt when you get into a high current situation, at least where we were, it is not for very far. Usually in a cut or some place like a bridge where the channel narrows. The problem then isn't one so much of speed but just fighting through it. There is nothing that I experienced on any of the lakes with wind that is like it. The closest thing I can think of is walking at an airport on one of those moving walkways against the walkway's direction or trying to climb an escalator against the direction it is running. It is not a good feeling when all of a sudden the whole boat is just being carried towards a non-moveable object and the outboard doesn't seem to be doing much.

We were going out under a bridge at one point where the channel narrowed and the tide was coming in. The current must of been 3-4 knots. I was at full throttle for a couple hundred yards hardly making any way forward. I know that if we would of had the 5 HP on then we probably might of had to wait.

Yes we are smarter now and do read the tide table, but also peak currents can vary in different cuts depending on how much water is inland that is trying to get back out and not necessarily pinned right to the tide table. Also depending on the time of day, getting dark or impending weather conditions you might not be able to move through some of these areas at the optimal time.

I think the motor and prop deal is trying to give yourself a larger safety margin when $hit happens. I know we disagree on this but the higher thrust prop on the same outboard does help you going forward also in those times you do find yourself being swept towards something you don't want to hit. You don't or can't be at hull speed in all of these situations and when you need a sudden burst of acceleration the higher thrust and larger diameter prop helps.

I don't think the Pacific in the area of southern Calif. presents these problems with tides there. The northwest and some of the northeast has even way bigger current problems than in Florida. In Florida the tides aren't high but the problem there is that down in the Everglades and other places there are thousands of acres in inland waters where all of that water has to come in from the ocean and back out again through narrow cuts twice a day. The height of the water inland might only be a foot or two, but the volume is unbelievable. Here is a clip at Everglades City showing the speed of the flow there at one point in time and other places it is a lot higher than that...


That is the Barron River but instead of the river flowing out to sea that is the tide coming in. That happens twice a day in both directions and sometimes more and sometimes less depending on the height of the tide.

The more I was in Florida the more I liked the fact that you aren't just navigating from point to point but also dealing with water depths that vary with the tides, currents that vary and all the different anchoring scenarios. It is a very dynamic situation. One that as an engineer I think you will really enjoy when you get down there with your Mac,

Sum
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Sumner, always good discussing stuff, even when we disagree..

But in this case again, I see again analogies (which generally can be misleading) and no measurements or physics. I’ve seen the video on the high current, why would anyone in their right mind take a displacement sailboat out in that?

We were going out under a bridge at one point where the channel narrowed and the tide was coming in. The current must of been 3-4 knots. I was at full throttle for a couple hundred yards hardly making any way forward. I know that if we would of had the 5 HP on then we probably might of had to wait.
For example the above just doesn’t mean anything without a measurement. I think that a six hp with the stock prop vs your 9.8 with the "high thrust" prop would have resulted in a speed of less than one knot difference. In both cases, the boat is at hull speed. If that less than one knot of hull speed from the excess hp is what it takes to make you comfortable, ok by me. But I think that is what this all boils down to for forward speed.

FYI, I’ve taken my boat up the Colorado river against the current flow. Hull speed is hull speed current or not.

This just doesn’t seem that complicated.. If two different outboards will get a vessel up to hull speed because of the physics of the bow wave and both speeds are within say 0.5 knot of each other, then both outboards will get the same vessel up to within 0.5 knots of each other if the vessel is placed in a moving body of water - i.e., in current (doesnt matter which way the body of water is moving with respect to the boat). I don’t think the theoretical hull speed is influenced by the fact that the body of water is moving with respect to land.

If this is not the case, please help me understand - with measurements and science, please, no analogies.
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
I think the motor and prop deal is trying to give yourself a larger safety margin when $hit happens. I know we disagree on this but the higher thrust prop on the same outboard does help you going forward also in those times you do find yourself being swept towards something you don't want to hit. You don't or can't be at hull speed in all of these situations and when you need a sudden burst of acceleration the higher thrust and larger diameter prop helps.
This also doesnt really fit with my expereince. For one month when my outboard was new, I had on the highest thrust prop available for several weeks and then a very standard prop and spent a fair amount of time comparing things. It was a controlled experiment, the only thing that changed was the prop. My peak forward speeds between the two props only varied by about 4%. Acceleration would be a little different as the you can get more HP out of the high thrust prop - which turns into a larger bow wave once your at hull speed but certainly would have a slightly higher acceleration if the boat is going at less than hull speed. Now we are not talking about reverse here as there was a huge and remarkable difference in how well reverse worked.

For the orginal poster, my choice between the two outboards would depend on if you want a faster sailboat or a more comfortable RV sailboat. Not a dang thing is wrong with going either way. A setup that sort of apeals to me would be a 26D with a rotating mast, good underwater foils, over all light but especially light on the transom. For this boat, the very largest outboard I would consider would be 6 hp and the reason is completely weight. As long as it was reliable, this would not influence me taking the boat somewhere that there was current. I think I could deal with the likely 0.5 knot less speed I would likely end up with wot.

However, on my 26S, I did get the 9.8 with the electric start. At the time I bought the outboard, I was more in the RV / Trawler mode and as someone approaching 60 with some joint problems, the electric start is nice.

If I bought again now, I would probably go with the 6 hp as Ive been moving my 26S more towards the sailboat end of things - based on the way I use it. Reliability between the two options would also be way more important than some future trip that might involve current.
 

Kestle

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Jun 12, 2011
702
MacGregor 25 San Pedro
I have a motor halfway between the two you are considering, and would ask if you get short steep waves where you are...

My Mac 25 does fine in them, but the six hp I had prior struggled with going up these waves, into the wind. On the East Coast, I wouldn't go near the Gulf Stream with under a 10 hp long shaft.

Just $0.02

Jeff
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Ask the same question on the power sailer forum.. You will get told you need at least 15 hp for that boat ;) (ok, maybe only by one person)

At the lake I sail on, I see considerably heavier boats than the Mac 25 with 2 - 4hp outboards (must also be two stroke, cant have the weight of a four stroke)..
 
May 4, 2005
4,062
Macgregor 26d Ft Lauderdale, Fl
My worst case scenario, would be a strong wind and current flowing against a closed bridge. add in some wakes and rain and a tight channel with rocks sides.

if I get turned sideways with the wind, and the current gets me, it takes me a long way/time to get turned back around.

The low pitch and bigger diameter props get the best bite and allow you to accelerate without so much cavitation.

if you can find a 6hp that can swing as wide diameter that would be the ideal.

IMHO.


- Hey Sumner, what do you figure the current is running there?
I'm thinking 3-4knts?
 
Mar 20, 2012
3,983
Cal 34-III, MacGregor 25 Salem, Oregon
Maybe I just don’t understand.. but why again in this thread is current an issue. On these relatively light 25-26 foot boats, between 6 and 10 hp with the highest pitch to grittiest lowest pitch props, the boat still gets to the theoretical hull speed range so the peak speed you achieve might vary by 1 mph.
its highly possible you didnt catch all of it :D, but in my opinion, I think current IS very much an issue in this conversation when one is talking about how much motor it takes to push thru high wind.... when there is a head current ADDED to the head wind, the speed of that oncoming current DOES matter...

although these numbers are very realistic, theoretically speaking, with barely enough motor to push the hull to its speed of 7kts,... the motor will make this 7kts when its calm, but add high wind and the motor barely makes 5kts against its strength..... then when you add 4kts of oncoming current, you are going to be there a very long time at 1kt... take away the current and you will be on your way at 5kts again..... remove only the wind and you will make 3kts..... remove both and your motoring at 7kts again.... current matters when you are sailing/motoring in an area that has it....

and as easy and convenient as the tables are to read, our world of leisurely boating does not always keep the same schedule as the tides and currents.:D
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
Walt I'm not interested in trying to go hull speed and never made a reference to it I believe. I'm interested in a motor/prop combo that will handle what MrBill talked about and keep the boat responsive in those situation. Sure I could probably put our 3 1/2 HP on and make it but it sure wouldn't be fun.

No I can't put this all in classroom math formulas. Just personal experience from lake sailing and our experiences of about 2 1/2 months on the water in Florida in different conditions.

The video at Everglades city was taken from where we tied up for a couple days at the Rod and Gun Club. You take that waterway in and out of there and boats are using it all the time. Sure you can try and time things perfectly sometimes but not always. Like I said you can find tide events on you $100 used Garmin but that doesn't equate to when the current will be the worst at any particular time and point between say Everglades City and the open Gulf about 5 miles away.

By the way since when are analogies not permitted :confused: Sure I gave some but some of what I gave was real life experiences ;),

Sum

P.S. I do believe that I told the O.P. that a 6 HP would be fine for him :)

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walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
We were going out under a bridge at one point where the channel narrowed and the tide was coming in. The current must of been 3-4 knots. I was at full throttle for a couple hundred yards hardly making any way forward.
You were going "hull speed" in the water around your boat in the above reference.. No?

I dont doubt at all that current is very important. My point is that between a 6 hp and a 10 hp, if you are going balls out (full throttle) like in the above reference, the difference in forward speed IN CURRENT or not in current - will likely be less that 1 mph, could be as small as .5 mph. That is just no margin at all (unless you really live on the edge)..

In the situation Mr. Bill pointed out, I was wondering if being able to rotate the outboard thrust would be the best solution.
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
You were going "hull speed" in the water around your boat in the above reference.. No?.....
No, or we would of been going 3-4 knots over ground and we weren't.

Another analogy and I know you hate them:

We have a full size pickup (V-8) and a mid-size pickup (4 cylinder) they are both geared so that 90 mph is top speed at say 5000 rpm and the rev limiter (hull speed). So they can both run the same top speed. Now put a 5000 lb. trailer behind each and go run them down the flat interstate at 70 mph. They can both do this. Then they come to a long grade. Which one is going to get to the top first even if neither can maintain 70?

The 9 HP with the big 4 blade prop is the 8 cylinder and the 6 HP with its little 3 blade prop is the 4 cylinder,

Sum

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May 4, 2005
4,062
Macgregor 26d Ft Lauderdale, Fl
In the situation Mr. Bill pointed out, I was wondering if being able to rotate the outboard thrust would be the best solution.
I have an old 8.9 merc, with a high pitch prop. -it doesn't work in reverse very well. (and can cavitate anytime, with a twist of the throttle)

If I was heading towards a closed bridge and current was taking me towards it quickly. I would spin the boat 180 degrees and face into the current/ stern to the bridge and go forward fast enough to maintain position or move away from the bridge. (assuming the 5 other boats waiting, will give room)...


*prop size is more important than max hp, but the smaller engines (normally) limit the prop diameter.

-The other issue we regularly beat to death is shaft length.
In a choppy sea the engine is moving up and down vertically so much its hard to keep the prop in the water and get 'traction' so that's where the longest length is important.
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Ok.. I think you were going "hull speed".. I think I will just have to stop by your place sometime and we can discuss, Beer might help me understand the analogy ;)

Mr Bill, you just need some hungry Crocodiles lining the bank to complete your picture. I might want the 50 hp if I had to do that all the time.
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
These discussions on outboards always have us in 6 knts of current with a 30 knot head wind, 3 foot choppy waves with a bridge that is going to clip off the mast.

What about the sailing aspect of a very light transom and a smaller outboard (still a long shaft or XL shaft). One manufacture has a carbon fiber transom to keep the weight low. Some racers remove the outboard from the transom and store it in the cabin. On another forum, a guy runs the tiller by remote control up at the front of the boat just to keep the back end light. Anyone lightened up the transom and had any sailing improvement?

The picture is of my 15 foot experimental boat. I like having some sort of motor on it to get away from the dock. I’ve seen other small sailboats take forever to get away from the dock because they had no motor. The long line of power boats that were waiting probably were not too happy..

I use this electric trolling motor because it only weighs 13 pounds. No other reason really. I’m also trying to get its weight as far forward on the transom to shorten the "lever arm". The battery is down low just under the mast and helps stability as ballast. This boat definitely benefits from a very light transom..
 

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May 4, 2005
4,062
Macgregor 26d Ft Lauderdale, Fl
Well it was worst case situation... :stirthepot:

but just for giggles & grins, most of the ICW from hollywood north to jupiter runs at around 1-2 knts maybe 3knts on a flood tide right near the bridge as it narrows.. those bridges only open on half hour. so there are lots of sailboats hanging waiting for it to open.

On the New river in FLL, 3-4 knots can be found at times. & about 4 bridges there to get out, but they are on-demand... so short waits (5 min max). but its narrow and shiny yachts line both sides.


The thing is, the current or winds can turn the boat, and getting it back is when the low pitch prop really would help.

I've had that happen, and it scarred me, so I'm careful not to be in that situation. -again.

if the OP was only on Lake lanier, no problem with the 4hp. but when he said west coast of Fl, tampa/ bradention that changes things.

personally, I have a lot more confidence in sailing the boat in/out of a channel than motoring...


Finally, those old 2 strokes are in demand for small keel boat racers. Like walt said, they go on the cabin floor when racing, and the light weight is key.

(I raced against a j27 with a 6hp 2 stroke, but it only used the engine after the boat was in the inner harbor, and sails were down... aka: flat water.)



-YMMV.
 

Piotr

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Dec 6, 2010
848
MacGregor 25 Rock Hall, MD
Sorry, but now I don't get it. Logically speaking, Walt is correct in the "current only" situation - If two engines will get you up to the hull speed, then well... "size doesn't matter." the hull speed is the speed against water, so if the current is 3 kts and you move at a maximum speed of 6 kts (hull speed), then adding a bigger engine does absolutely nothing (6kts-3kts=3kts with either engine). You may change the maneuverability with a different pitch propo, but that's it. Unless you add an engine big enough to get on the plane (or add a midship step to the hull).
Now, when the wind is concerned, a bigger engine would be of a definite advantage, because you are pushing the boat against air, and the hull speed does not matter. Or am I missing something?
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
Is it the analogy that is confusing.

Sumner, hope you dont mind my slightly altering your analogy to show why I dont like them..

Sumners analogy
We have a full size pickup (V-8) and a mid-size pickup (4 cylinder) they are both geared so that 90 mph is top speed at say 5000 rpm and the rev limiter (hull speed). So they can both run the same top speed. Now put a 5000 lb. trailer behind each and go run them down the flat interstate at 70 mph. They can both do this. Then they come to a long grade. Which one is going to get to the top first even if neither can maintain 70?

The 9 HP with the big 4 blade prop is the 8 cylinder and the 6 HP with its little 3 blade prop is the 4 cylinder,
Welll, heck.. the 8 cylinder car will go up the hill faster..

New analogy..

Same two vehicles on a flat highway can maintain and are also limited to 70 mph, both pulling trailers.

Now we are going to throw in current. But current is like a people conveyer at an airport. If you stand still, you move at the rate of the converyer. If you walk in the opposite direction of the conveyer, your speed with respect to the ground is slower than you are walking. Walk at the speed of the converyer in the opposite direction and you dont move at all.

Now take both of those vehicles going 70 mph and put them on a giant conveyer doing 70 mph. If they go in the direction of the conveyer, they will do 140 mph. If the go opposite, both will stand still relitive to the outside ground. Both still have the same speed at all times to an observer on the ground (not on the conveyer).

Slightly different analogy but completey different conclusion - and why I dont like analogies.

Ive already posted a bunch of times why if both a 6 hp and a 10 hp get you close to hull speed, the speeds of both setups will still be about the same in current. The physics of hull speed are local to the body of water the boat is moving in. Hull speed in a body of water would not be influenced by that body of water moving with respect to land.
 

Piotr

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Dec 6, 2010
848
MacGregor 25 Rock Hall, MD
Is it the analogy that is confusing.

Sumner, hope you dont mind my slightly altering your analogy to show why I dont like them..

Sumners analogy


Welll, heck.. the 8 cylinder car will go up the hill faster..

New analogy..

Same two vehicles on a flat highway can maintain 70 mph, both pulling trailers.

Now we are going to throw in current. But current is like a people conveyer at an airport. If you stand still, you move at the rate of the converyer. If you walk in the opposite direction of the conveyer, your speed with respect to the ground is slower than you are walking. Walk at the speed of the converyer in the opposite direction and you dont move at all.

Now take both of those vehicles going 70 mph and put them on a giant conveyer doing 70 mph. If they go in the direction of the conveyer, they will do 140 mph. If the go opposite, both will stand still relitive to the outside ground. Both still have the same speed at all times to an observer on the ground (not on the conveyer).

Slightly different analogy but completey different conclusion - and why I dont like analogies.

Ive already posted a bunch of times why if both a 6 hp and a 10 hp get you close to hull speed, the speeds of both setups will still be about the same in current. The physics of hull speed are local to the body of water the boat is moving in. Hull speed in a body of water would not be influenced by that body of water moving with respect to land.
Sumner's analogy is flawed, because if both cars have the same torque (prop pitch), I believe they'd get to the top at the same time.
 
May 4, 2005
4,062
Macgregor 26d Ft Lauderdale, Fl
I agree, with the logic. it makes perfect sense. you can only go hull speed. Agreed/

but in the real world the current, and winds especially in confined spaces, change or vector quickly and without warning.

So you are making 3knts against a current, and you are heading right into it , but then it shifts, (or the wind does), and pushes the bow to one side. you have about 2 seconds to correct or, the boat will be broad side, and then it will be trouble.

That river Sumner posted is protected, but it traverses unprotected bays, on the way out. if you were fighting to go up river with a gusty cross wind, you really need to be alert.


(late post)...

I think current is NOT constant in cuts, channels, and rivers.
Due to the bottom and change in widths

and if you punch it on a small prop, it cavitates more. so you have to apply throttle slower to get the bite and turn the boat.

in a larger bay, like tampa bay, or charlotte harbor its pretty consistent, until you get to boca grande. where it picks up, like a river...