High thrust props and sailboats

Aug 7, 2011
496
MacGregor 26S Lakeland, FL
So i had read this post and started to really wonder what prop my Frankenmotor had on it. I went and took a picture... 11-3/4 x 9-1/4!!
Way oversized for the 9.9 four-stroke that i have. No wonder i never got rpms up very high, even though the boat would get to hull speed. I suspect my fuel economy will improve when i can get the motor up into higher power range. I'll be looking for a smaller prop... What diameter do you suggest? 10"? 9"? And it sounds like a 6 or 7 pitch... so something like a 10x6 or a 9x7 maybe?
 
Aug 1, 2011
3,972
Catalina 270 255 Wabamun. Welcome to the marina
wow. A lot of techy stuff for a boat that with the stock, factory prop on the Honda BF8 would push it to hull speed at 2/3 throttle all day. I don't see any advantage to a high thrust prop at all. :)
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
I don't see any advantage to a high thrust prop at all.
I dont either unless you need that reverse..

Sounds like your prop probably was intended for a higher HP outboard. You could probably find what prop came stock with it. It may not be "high thrust" but it just doesnt matter a whole lot using a 9.9 hp outboard on this boat (more hp than you really need). You might have fun just trying the high thrust prop.. Just be careful of damaging the reverse lockout..
 
Sep 20, 2014
1,320
Rob Legg RL24 Chain O'Lakes
wow. A lot of techy stuff for a boat that with the stock, factory prop on the Honda BF8 would push it to hull speed at 2/3 throttle all day. I don't see any advantage to a high thrust prop at all. :)
So how fast will it push it into a 40 mph headwind against a 2 knot current? There are times when you need to make sure that every last bit of HP is put into moving the boat forward. Under normal conditions, I hit hull speed at about half throttle. But last time out, I was at full throttle, the engine was turning about half speed, and the boat was only moving about 1-2 knots. I was running with the swing keel part way up for less drag. Straight down wind was lined with large rocks.
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
how fast will it push it into a 40 mph headwind against a 2 knot current?
exactly 2 knots slower than 40 mph headwind with no current. Oh no.. current discussion again..

Pointing out that you have a 4 hp with a boat that is about the same size. All this discussion is with closer to 10 hp on these fairly light boats.
 
Aug 1, 2011
3,972
Catalina 270 255 Wabamun. Welcome to the marina
The MacGregor doesn't need a 9.9, and under most circumstances, it doesn't need a high thrust prop or a fleet of trolling motors to get someplace. Any argument can be countered with extraneous events. All I'm saying is that we had a 26c with a stock 8 hp Honda for 15 years, and 99.99999% of the time it was just fine.

As to the 40 mph statement, have fun. I wouldn't have been out in it. You tend to break keel cables in pounding water.
 
Aug 1, 2011
3,972
Catalina 270 255 Wabamun. Welcome to the marina
It was on the trailer dragging the motor up the ramp. Oops.
 
Sep 20, 2014
1,320
Rob Legg RL24 Chain O'Lakes
exactly 2 knots slower than 40 mph headwind with no current. Oh no.. current discussion again..

Pointing out that you have a 4 hp with a boat that is about the same size. All this discussion is with closer to 10 hp on these fairly light boats.
No, the Mac weighs twice what my boat does, with considerably more freeboard. Looking at worst case scenario. Some lakes storms can develop rather quickly.
 
Aug 7, 2011
496
MacGregor 26S Lakeland, FL
So how fast will it push it into a 40 mph headwind against a 2 knot current? There are times when you need to make sure that every last bit of HP is put into moving the boat forward. Under normal conditions, I hit hull speed at about half throttle. But last time out, I was at full throttle, the engine was turning about half speed, and the boat was only moving about 1-2 knots. I was running with the swing keel part way up for less drag. Straight down wind was lined with large rocks.
I think one of walt's main points in his detailed analysis was that it's of some value to make sure you're able to get up to full engine speed. The motor, be it a 4 HP or a 9.9, only makes that rated HP when somewhere very close to full RPMs. If your motor was turning only half speed at full throttle, then your prop is too big. (Just like mine is.) I may only be getting the benefit of about 5 HP out of the available 9.9 HP because I can't get the engine up to the speed where it is producing full rated HP. So I want to find a prop, call it high thrust, that could consume full engine HP while turning at full speed.

How fast you actually go thru the water has a lot to do with the windage, drag from stuff growing on the bottom, etc.
 

Fred

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Sep 27, 2008
493
Catalina 28 mkii 745 Ottawa, Ontario, CA
It was explained to me that the ventilation holes in the four blade prop shown is to improve the reverse thrust. The holes provide a path for the exhaust gases to pass through to minimize cavitation. Is that what the "high thrust" refers to?
 
Aug 7, 2011
496
MacGregor 26S Lakeland, FL
So i thought my prop was oversized, as i posted earlier. My motor was rebuilt with a little of this and a little of that by a local mechanic who does this type of thing, so the model on the nameplate doesn't match the way the motor is actually built. Because of that, I presumed the prop was just a secondhand find that the guy used, but i am wrong, apparently. The prop i have is the size that came on the high thrust Yamaha 9.9 motor at that time.
Walt, you said the prop you were running was 8.5" diameter? Sumner, your 4-blade, 7-pitch is what diameter?
AndyVS mentioned a 10x7 prop on his motor.

My prop is way bigger, and more aggressive in pitch, than any of these! Do you think it's possible that Yamaha just expected the motor to not ever turn at full speed with the large "high thrust" prop? That seems odd to me. Should I expect to be able to get up near full motor RPM?
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
You have to take into account gear ratio also. All the my experience in this this thread is with 2.08. RPM in the range of 5000 to 6000 for full HP seems to be common with most of these outboards but the gear ratio varies. Slip calcs also take into account gear ratio. Do you know what yours is?
 
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Aug 7, 2011
496
MacGregor 26S Lakeland, FL
You have to take into account gear ratio also. All the my experience in this this thread is with 2.08. RPM in the range of 5000 to 6000 for full HP seems to be common with most of these outboards but the gear ratio varies. Slip calcs also take into account gear ratio. Do you know what yours is?
No, but that came to mind also. I'll have to look and see if there is a way to tell what it is. If not, i may remove the spark plugs and turn it over (with the cord) while in gear to see what it is...
 

Sumner

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Jan 31, 2009
5,254
Macgregor & Endeavour 26S and 37 Utah's Canyon Country
Aug 7, 2011
496
MacGregor 26S Lakeland, FL
8.7 inches

Sumner
I never responded to all of this, but thanks to everyone for letting me know what size props you are running!
I just re-configured my throttle. May purchase one of those little battery-powered tachometers this fall to see what speed the motor runs with my big prop.
 

walt

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Jun 1, 2007
3,511
Macgregor 26S Hobie TI Ridgway Colorado
If you get one of those small tachs..

I had this Kedsum except that the one I had did not have the replacable battery. It sort of worked on my Nissan 9.8 but I didnt think it was very reliable. I could not get it to work at all on my Susuki 2.5. I probably would not buy it again.. https://www.amazon.com/KEDSUM-Water...rd_wg=0JlxE&psc=1&refRID=2K8G747W8DAZWQK1Z9PH

If I were going to buy another (and I might), I would try this Runleader https://www.amazon.com/dp/B01FZ3YMOQ/ref=psdc_15729791_t1_B01E16UYFE

Reason I would try this one is that the method used to pick up the spark signal "seems" to me that it would be more reliable in that it has a seperate ground that you attach to the outboard chassis somewhere. It doesnt need a battery which if it does work is nice. Reviews on it seem OK.

I almost bought that Runleader yesterday.. but didnt. I know on the Kedsum you can "program" it for 2 or 4 strokes (ie, one pulse per revolution or one pulse every two revolution). You have to somewhat try the different settings to get one that makes sense which can be a little difficult if you think your outboard is not set up right. For example, if the outboard were running well and you were measuring for example 2600 RPM, you likely have the incorrect setting for pulses per rev and when you tried a different setting, you would see 5200 RPM which would make more sense.. I dont even know if the Runleader can be programmed for 2 or 4 stroke.. you may also just have to double the number you read (or something like that).

edit.. how is your Russian? Here is a video of the install. It must have an intertnal non replaceable battery

If you buy another one (before I do) and it works well, please let us know..
 
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