high thrust prop

[katana750]

I know this has been talked about before, but I cant seem to find it or maybe somebody can help me with the use of this sites search engine I always have zero success with it.. I usually use google to find things on this site.

I typed in high thrust prop...I know there is way more info on this site

I bought a new rudder for the boat and am not interested in chewing it up like I do now.

I thought it would be good idea to buy a prop guard and before I do that I thought it wise to get a new prop first.

I have a yamaha 9.9 two stroke 1993. It currently has a three blade prop. What I was reading before was that a four blade high thrust prop would work better. They seem to come in different pitches.

It would be great if some body could just tell me what works well for them and were you bought it and I could just copy. It seems to be my favorite technique for boat modifications. One day I might actually have an original
idea to post. Until then....

Thanks again

Dave

 

[DJN51]

Try Boats.net ,they carry high thrust props for your motor,Have one on my 6 hp 4 stroke,works great

 

[katana750]

Thanks that is a good resource. What sort of pitch should I be looking at. Just out of curiosity by decreasing my pitch and using a four blade pitch prop could I run at a lower rpm and still maintain a 5 knot speed. I guess what I am asking is what happens to the fuel economy by changing the prop? I would guess it would be slightly better, but it is better to ask. How much of a change to the prop can I make before it starts becoming hard on the motor?

Thanks

 

[walt]

Props have been discussed here a lot (possibly why no posts) - use the search in the blue bar at the top.

One thing I hope you can do first for our benefit - before you get a new prop, take your boat out with the old prop and measure your boat speed with the throttle wide open when there is no wind and no current. If you have any way to do so (most of us don’t), also measure rpm. Then when you get the new prop, do the exact same thing.

Reason I suggest this is that I did the above with the two props shown at the bottom of the post and got nearly the same exact speeds, just way different engine rpm. We have nearly the same setup (26S, just under 10 hp) so its a boat that needs maybe 5 hp to get up to hull speed so any additional hp past that just gets wasted creating a bow wave.. Hull speed is relative to the water around the boat so this also means your boat speed for both props will also be close to the same even in current.

Some general things about props that I think are true (not a prop expert so keep that in mind)

* swept back blades are more efficient going forward but are much worse in reverse..

* all high thrust props I have seen have symmetrical blade shapes and while they are likely slightly less efficient in forward, they work way-- way better in reverse.

* to increase thrust, you either need to move a larger mass of water or accelerate the mass of water faster. So more blade area would move a larger mass of water and a higher pitch (more angle of attack) would accelerate more water. So for the same thrust and boat speed, more blade area results in lower rpm and higher pitch results in lower rpm..

In my opinion, the main reason for going with a high thrust prop on a boat like a 26S and a 9 or 10 hp outboard (ie, you have excess hp so dont need to get to high rpm to get the peak hp available) is to get the much better reverse. If you are looking for efficiency and gas mileage for long motoring, your original three blade swept back prop is going to be better.

Also in my opinion, if you can rotate the outboard, you can run the boat slower when maneuvering into a dock or slip and the swept back prop works just fine. It won’t stop the boat nearly as fast as the high thrust prop but I get by just fine with a rotating outboard and the swept back three blade. I take the tradeoff because I want the efficiency and lower noise of lower rpm in the forward direction.

However, if you cant rotate the outboard and need to run the boat a little faster to control it when docking, I think a symmetrical blade high thrust prop would make life a lot easier and you can come in to a slip fairly fast and then very quickly stop the boat. The high thrust reverse is a huge amount better.

The two props I have used on my boat are shown below. The prop on the left is 8.5 pitch, the prop on the right is 5 pitch. The 5 pitch four blade prop resulted in obnoxiously higher rpm and vibration for motoring but it had an amazingly powerful reverse. Some outboards are not designed mechanically to handle a lot of reverse thrust so you have to be careful.

I did not like the high forward rpm of the 5 pitch four blade (noisy and more vibration, almost for sure worse gas mileage) and have been using the three blade 8.5 pitch as its more efficient in forward and since I can rotate the outboard and its not a heavy boat, the reverse works fine.

Sumner uses the four blade with I believe a 7 pitch and I believe really likes it. I have not tried this prop but if the reverse is important to you, that is about the prop I would also go with.

I think the Yamaha also has a high thrust three blade prop and I’m not sure of the pitch but you notice that the blades are symmetrical so it should also have a great reverse. Props that work well in reverse may also have better ways to handle the hub exhaust when going in reverse.

 

[DJN51]

Agree with Walt,my High thrust prop doesnt help with speed much over OEM prop,but in manuvering at slow speed what diff,I can back out of slip at idle,the prop exhaust is so confined you are always in uncavaitated water.

 

[tkanzler]

Agree with Walt,my High thrust prop doesnt help with speed much over OEM prop,but in manuvering at slow speed what diff,I can back out of slip at idle,the prop exhaust is so confined you are always in uncavaitated water.

Agreed. I used a high-thrust prop on my 2-stroke 9.9 Evinrude a couple of boats back, and the difference in reverse was very noticeable. But so was acceleration, or perhaps more accurately traction, at least when punching through waves, which was a common problem in the marina I was in at the time (breaking waves in the tiny inlet).

I tried a 5" pitch three-blade on that same OB, on the assumption that a low pitch was most appropriate for a slow boat, but the engine would overspeed too easily and the 'traction' was no better than the one that came with the engine (can't remember what it was, though).

The answer seemed to be more blade, which in my case was a high-thrust prop. I use a 4-blade on my 50 hp for towing a wakeboarder, and the top end seems to be about the same as the 3-blade, but with one less inch of pitch on the 4-blade. Reverse is also improved. FWIW.

OMC 10" x 7" pitch high-thrust (no longer made, I believe):




Michigan Wheel 10" x 5" pitch:

 

[walt]

I had watched both of the props in the previous post for a fair amount of time. I think the four blade has slightly more area and it did produce more thrust in forward (which just creates a large bow wave once you reach theoretical hull speed) but I really think the main reason was the higher rpm. If you have a prop that for any reason doesnt allow the outboard to get to its rated rpm, it wont reach peak hp. Higher rpm no doubt gives more punch, more hp - but if you have listen to the noise for very long such as even motoring for 1/2 mile.... well.. I got tired of it.

I really dont understand what more prop area does.. maybe it has something to do with Reynolds numbers or something and efficiency? Im pretty sure that if you increase the area of the prop by 35%, that you do not get a corresponding increase in hp of 35%. However, you might run the prop at 35% slower rpm to get the same hp and corresponding delivered thrust??

 

[doehunter]

I had watched both of the props in the previous post for a fair amount of time. I think the four blade has slightly more area and it did produce more thrust in forward (which just creates a large bow wave once you reach theoretical hull speed) but I really think the main reason was the higher rpm. If you have a prop that for any reason doesnt allow the outboard to get to its rated rpm, it wont reach peak hp. Higher rpm no doubt gives more punch, more hp - but if you have listen to the noise for very long such as even motoring for 1/2 mile.... well.. I got tired of it.

I really dont understand what more prop area does.. maybe it has something to do with Reynolds numbers or something and efficiency? Im pretty sure that if you increase the area of the prop by 35%, that you do not get a corresponding increase in hp of 35%. However, you might run the prop at 35% slower rpm to get the same hp and corresponding delivered thrust??

I think you are on the right track with the area thing.
You could compare it to an airplane wing.
If a loaded airplane has too small of a wing, it won't fly no matter how much angle of attack there is. (nose pointing up)
A boat prop will not be efficient if it's too small.
Soooo, with a given pitch, if you start off too small and just increase prop size, you would see an increase in speed until the get to where the prop is as good as it gets. Then you would only see an increase if you go up in pitch.
As has been said, a big prop with low pitch gives you more pulling power but less speed.
I smaller prop with a higher pitch gives more speed but less pulling power.
Personally, My 26S runs pretty nice on my Honda 8hp. I don't know what the prop is, but I assume it's just the stock prop.
It'll run near hull speed at about a little over half throttle. Full throttle makes it a PITA to listen to, and not much increase in speed.

 

[katana750]

Thanks for the responses thats what I was looking for. I have found the four blade 5 pitch prop, but I was wondering what that would do to forward motion noise and fuel economy. You have helped narrowing down my search. I was actually pretty happy with what I have although I run about 5 knots at half throttle. I was just wondering if I could do a little better before I put on the prop guard and replace the propeller I have now with a big chunk out of it.

Props have been discussed here a lot (possibly why no posts) - use the search in the blue bar at the top

I know there has been a really good discussion about this in the past. I have seen it but when I search for it using this site search section. Not much comes up I know it is there...its a bit frustrating.... I have had this problem with other topics as well. I always have zero success. I have even tried it on recent topics. It just doesn't seem to be a very effective tool.

If I type in high thrust prop I only get one vague hit. Any tips for more success?

 

[katana750]

I think the Yamaha also has a high thrust three blade prop and I’m not sure of the pitch but you notice that the blades are symmetrical so it should also have a great reverse. Props that work well in reverse may also have better ways to handle the hub exhaust when going in reverse.

Dumb question but when you say the blades are symmetrical what does that mean and how does that make it better for reverse? I just don't know what I should be looking for. The pitch is clear...but I am not sure what to look at for symmetrical.

My first response is aren't all props symmetrical....But I am sure this is not what you are trying to explain to me.

Thanks
Dave

 

[katana750]

Thanks for the responses thats what I was looking for. I have found the four blade 5 pitch prop, but I was wondering what that would do to forward motion noise and fuel economy. You have helped narrowing down my search. I was actually pretty happy with what I have although I run about 5 knots at half throttle. I was just wondering if I could do a little better before I put on the prop guard and replace the propeller I have now with a big chunk out of it.

Props have been discussed here a lot (possibly why no posts) - use the search in the blue bar at the top

I know there has been a really good discussion about this in the past. I have seen it but when I search for it using this site search section. Not much comes up I know it is there...its a bit frustrating.... I have had this problem with other topics as well. I always have zero success. I have even tried it on recent topics. It just doesn't seem to be a very effective tool.

If I type in high thrust prop I only get one vague hit. Any tips for more success?

I think my problem was I have been using the search section in the light blue bar instead of the dark blue....I would get frustrated and switch to google which usually works.

 

[StinkBug]

Take a look at the 2 props in tkanzlers post. on each of them draw a line from the center of the prop directly down the middle of one of the blades. Now look at the section of blade on each side of that line. Notice how on one prop the blade itself is symmetrical and on the other the blade is kind of skewed to one side. The one that is symmetrical will work roughly the same when spinning in either direction, the other is optimized for one direction of rotation over the other (forward).

 

[walt]

Yes, it’s the search in upper dark blue strip that woks (I’m pretty sure)

I’m not sure if there is an official definition of symmetrical but Stinkbug defined what I was referring to. I think the big reason to get a prop like that is the great reverse and if you pick the right pitch (not too low of a number for our boats since they are on the light side), the noise and vibration won’t be bad (Im guessing since I have not tried a higher pitch high thrust prop). If you cant rotate the outboard or don’t want to, I think the ability to just give a quick burst of reverse and pretty much stop the boat would make docking easier as you could be more comfortable keeping the boat speed up a little bit so that the rudder isnt stalled all the time. But.. the high thrust just didn’t solve any problem I had (I can rotate the outboard) so I’m using the higher pitch three blade swept back. I tend to use the boat in a "trawler mode" a bit so picked the tradeoffs favoring long motoring.

I also have a prop guard and one thing I noticed after getting the one shown in the picture is that my zinc was deteriorating faster after put on the guard. I think it forms a galvanic circuit between the zinc and stainless steel guard so if you get one of these, you might want to keep it out of the water as much as possible. The guard is only just to keep the prop from munching the rudder when the rudder is angled up some.

If you get a high thrust prop, take some before and after speed measurements, I think it would be interesting to everyone. I sort of wish I had a way to measure rpm - but not enough to spend any money..

 

[Cruiser1]

These may help for those interested.

Brief but complete outline of propeller terminology: http://www.halepropeller.com/PropellerTerminology.html

Detailed discussion on propeller terminology and usage: http://argonaute.pagesperso-orange.fr/Helice.pdf

 

[Chris & Lenore]

Just to add to this thread - I checked the specs for the Yamaha 9.9 HT and regular models and the gearing is different on the HT model in order to keep the rpm range vs the speed range the same.

I have the HT model and it has incredible "traction". Reverse is awesome due mostly to the exhaust dispersion ring.

The only negative is that I sail in a weedy area and it chokes worse than the regular prop engine I used to have. Those four big blades just seem to grab and retain more weeds.

Here is a question - has anyone seen devices to cut weeds (and line) for an outboard? I have seen numerous contraptions for inboard.

Chris

 

[walt]

The weed shedding mentioned in the link for the swept back foil was interesting, looks like another trade off (weed shedding) to add to the list. Always tradeoffs.

But.. I know you can still get a docking line really "nicely" tied up in a swept back foil prop at a really bad moment when your drifting towards some big rocks in high winds. I’ve caught weeds on the prop guard and even on the prop itself once or twice even with the swept back foils but it hasn’t been often enough that I even thought of it until mentioned.

 

[katana750]

Ok I am going to head out this weekend

I have found out I currently have a 3 blade 7 pitch prop.... obviously not stock

I am debating weather I should buy a 4 blade 7 pitch prop or a high thrust 4 blade 7 pitch prop or just stay with the one I have.....except the low speed maneuverability is a bit of a problem

The outboard is just a 9.9 Yamaha two stroke and it is not the high thrust model. They make one but only in a four stroke.

What I have noticed so far with the boat. I only reach hull speed at basically WOT. Mind you I drag a dingy. I will try with out this weekend. When the motor is cold I notice the motor will run at WOT at a higher rpm and then once it is warmed up I can here it drop its rpm a bit. Motor runs perfect though. My first thought was there was a problem with the motor, but after reading about prop pitches a more likely scenario may be the motor is overreving a bit and then automatically cuts itself back. I don't tend to run at WOT all the time but it does give me a base line. It would be nice to have a tach.

So here is my thought a four blade 7 pitch prop might be a perfect prop. With the extra blade the motor won't over rev and it should be better at lower speed and reverse.

The high thrust four blade 7 pitch. Might be designed best for a high thrust motor and might be a bit too much for my motor although from the other posts this would be the best for control. Both are Solas props

How is my thought process? Does my idea seem reasonable?

Yamaha has a dual thrust prop which sounds perfect, but it seems to only fit four strokes.

It would be nice to have a handful of props and try them all in one day and pick the best one. Thanks for helping me shave a ton off my learning curve.

 

[katana750]

Ok I just returned from the lake just in case somebody id following along
Wot is about 6.2 knots
.75 throttle 5.1 knots

Dropping the tender adds about .5 knots

I am a bit curious as to what a should expect for a good cruising speed. I have read several times about people reaching hull speed at half throttle. I can't reach it at full throttle.....keeping in mind 7 pitch prop. full boat, wife, kid, dog and dingy. (still don't reach hull speed when dingy is dropped.

Off to the web I go.

Dave

 

[katana750]

After looking at Walts past posts explaining his comparison speeds with a 5 pitch and 8.5 pitch prop (thanks for pointing out the second search link at the top of the page) I can't realisticly expect more than 6.5 knots And 5 knots seems a realistic cruising speed.

I thought I kind of had a handle on the different prop configurations. but I am still a bit confused on the high thrust def. When I go to Solas and compare the 7 pitch prop and the high thrust 7 pitch prop. From the picture they look like the same prop. With the high thrust being slightly bigger. The blades look pretty similar. Can't really tell if they are symmetrical either. I could always give them a call, but just wondering if somebody was using Solas prop.

 

[tkanzler]

You really need to take tachometer readings at WOT to know what the engine is doing, and compare that to the published design engine speed. You can't pick a prop without knowing what the engine is doing.

Solas shows a 10x5 and 10x7 4-bladed high-thrust prop for that OB, but without knowing getting a baseline on how its performing now, you can't know where to go.