Propeller shaping tips

[Ducksauce]

Hey guys, looking to get the most out of my Powertech SRA4 10x10. I use this prop because of the rules in the competition I race in (25hp inflatable rescue boat).

The prop comes quite heavy with a large blade surface area so I take back the leading edge by about 7mm especially near the root of the blade, from there I thin the leading and trailing edge down with a linisher to get a convex style blade cross section shape. I leave the cupping as is, there isn’t too much and it helps prevent ventilation when turning. the prop is then static balanced and polished.

All this work can usually gets 2-3 knots out of the prop, but this can be the difference between winning and losing when your max speed is 24 knots. Is there any other tips and tricks I can do to my prop that could help me get some more top speed? I have tired sending my props to prop shops with mixed results, there aren’t too many where I live.

[XstreamVking]

Have you tried cutting the barrel length down?

[LakeFever]

Have you tried coating the prop with lipstick and running it? The old school say wherever lipstick remains after a run should be cut off

[Ducksauce]

Thanks for the suggestions guys,

Cant cut down my barrel length, the rules don't allow any modifications to the hub of the propeller.

will give the lipstick trick a try, sounds like a good way to tell where to cut!

[Ribber]

Do you have some kind of video of such a racing event? So we can better understand how the engine and boat and crew and everything work together?

[Ducksauce]

Sure,

Here is a video of our new zealand competitors https://www.youtube.com/watch?v=D3ZnbsUbMdQ, in Australia we also incorporate a 360 turn before the patient pickup which really splits up the competition. Its a full throttle flick turn that requires a prop that grips but also ventilates at the same time. Because of this we keep the cupping on the blade as is but do not add any more.

[LakeFever]

That looks like a ton of fun!! !

[Ribber]

It sure does look like a ton of fun! I also think I'd be scared of my feet getting in the propeller if I was the one being picked up, but I assume you have a prop guard on it.

Well, in any case it was as I suspected that this is very much an acceleration and grip sport so I'm not sure how much how much more performance can be gained from fiddling with the prop. If you have enough resources (money) get another 3 props and shape them a little different and find out what works best with a stop watch, take one prop and keep cutting the blades down until the surface area gets too small and you lose grip in the turns and maybe top speed too. Then thin the blades out until they break.. then you'll know the limit and can do it a little less on the next prop. This is how many racers do it. And they have several of the "same" props but for whatever reason one often runs sweeter than the others and they have a favorite for different water conditions etc. If you're revving too high you can try cupping it a little bit more.

But it's also about the whole package so here are some ideas:

* Have your entire crew lose weight and trim as much as you can off everything. On such a small boat and crew about every 50 lbs saved should equate to 1 knot higher top speed and better acceleration.

* on flat flat water typically the further back the weight and the more of the hull is out of the water the higher the speed, at the cost of stability. In rough waters it might be the opposite and you want to fly level instead.

* If you want to experiment you can try different engine mounting heights, trim angles and also maybe a 5-10cm engine setback. On flat water the higher the engine mounting height without losing grip the faster.. typically. So maybe try this on one prop before cutting it down too much, more cup grips better for this also.

* It could be worth testing different pressures in the tubes to see what works best. Softer pressures can sometimes help absorb the waves better and make the boat faster. Or it can make the tubes and hull dig in and slow you down.

* Obviously try to minimize all water resistance under the hull, but also wind resistance above it.

* Don't forget your stopwatch and GPS since some things feel faster but are not.

Let us know if you have any luck

[ZUL8TR]

Thanks for the suggestions guys,

Cant cut down my barrel length, the rules don't allow any modifications to the hub of the propeller.

will give the lipstick trick a try, sounds like a good way to tell where to cut!

Make sure the lipstick is not the no smear type, I put it on the forward blade face and do metal removal on that face. The lead edge angle of the blade is very important to get the correct AOA for the water pickup and reduce separation at the lead edge.