Power prop on a sailboat

[phorvati]

Is there anyone using powerboat prop on a sailboat. I'd like to get a smaller propeller. I have 18-10 and barely get to 1250 RPM at full throttle. Feathering (new) are outrageously expensive, new campbell is still out of my budget, and used sailboat propellers are hard to come by. I wanted a 3 blade fixed (don't care about drag at all). I narrowed it down to 15/10 RH 3 blade sailboat prop, but even the cheapest new is still too much money. One prop shop technician said if you use powerboat 3 blade you'll have more surface area so you might want to lower the diameter or pitch when compared to 3 blade sailboat prop. They also give more drag which i really don't care about. But here in RI theres a bunch of 3 blade powerboat props in consignment in good condition for around or less than 100$. Are there any other reasons to not use the power boat prop other than drag?
Petar

[never monday]

It'll be like draging a 15" solid circular disk behind you. I think there is something else going on. What engine do you have? what does the boat weigh?

[seaclusion]

Using a 'powerboat' three blade prop will indeed cause more drag than a two blade 'sailboat' prop. You have to make the decision whether you want more thrust while motoring, as in trying to go against a foul current, or less drag while sailing to gain that extra 1/4 knot. That is strictly a personal matter.

If you chose to get the most thrust while motoring, first select the biggest diameter prop you can fit in the aperture while still maintaining a 10 percent diameter distance between blade tips and hull/struts/skegs. then select pitch based on cruising speed, boat displacement (actual weight while loaded for cruising and 2/3 fuel and water) engine shaft horsepower, and engine RPM. The major propeller companies will do this for free if you fill out their form and send it in. They are hoping to sell you a new prop. After getting their recommendation, you can adjust the pitch based on your experience with the boat under real world conditions.

If you choose to get a two blade 'sailboat' prop with a small blade area, you should still select a prop that will perform efficiently at the speed you intend to travel and modify the pitch based on your experience. Still select the largest diameter that can be accomodated since it is diameter (and blade area but you are not concerned with that in this instance) that is most important in transmitting power to the water. Pitch should be matched to the intended speed and engine RPM just as with a three blade prop. I dunno if I has helped ya or not, but this is probably the way to start the descision on what kind of prop.

Richard

[hellosailor]

"Are there any other reasons to not use the power boat prop other than drag? "
Yeah. If you buy a slow speed, heavy displacement hull, powerboat prop...you'll find it is the same prop at the same price as a "sailboat" prop. The prop doesn't care what your boat is, it is built to move a certain disaplacement, at a certain hull speed, at a certain engine RPM.
Putting on the wrong prop can be an expensive mistake, especially with fuel prices today. Find out what the right prop is (it may not be what you have, see Dave Gerr's propellor book or ask a couple of prop makers what they suggest for your boat) and then nag every used-prop-shop and chandlery (like Sailorman) in the country until you can find one at a price you can afford.

Or...you can use a lawnmower blade and an anvil, and try pounding it into shape. Certainly CHEAP.<G>

[phorvati]

I did ask many prop shop technicians. We arrived at 15-10RH sailboat 2 blade propeller. Since I'm trying to get the powerboat prop i'll make it 14-10RH 3 blade fixed RH.
I have atomic 4 with 2:1 reduction drive. I really need to get the RPMs up to 2000-2500 at full throttle. At moment I only get to 1250 and it is very fuel inefficient. I am probably developing 10HP or so. At 2k RPM I'll be in the neighborhood of 20HP which is much better. And i care far more about thrust under power then drag. I'll keep the transmission in neutral so the thing will spin when under sail as opposed to stay still.
Petar

[Alan Wheeler]

18 is a big prop for a small engine. Is it three or two blade??
I would get the prop cut down and repitched to suit your needs.
Another question, when you refer to "power boat prop", are we still talking a Bronze shaft driven aren't we? not an outboard?. Outboard would be a very bad choice.
Right now, there is a Morgan two blade 12RH14x 1 1/8" on your US Ebay site. It is in Florida and is $47.00 with two days left and so far no bids.
There is also a Gori 15x10 two blade folding on our NZ trademe site. Unfortunately it is LH. But some gear boxes can be run easily either way, so it could be just a case of changing direction. Starting bid is NZ$100 which is about US$60'ish and so far has no bids. It has a close date of Friday 1st. I can help you with that if you are interested in taking that further.

[hellosailor]

Petar-
" I'll keep the transmission in neutral so the thing will spin when under sail as opposed to stay still" I'll assume you've checked that your transmission won't be harmed by that. Some will be damaged, others won't, it varies by make and model.

If your engine is running that slowly...I'm surprised the prop shops didn't take that into account. IIRC the A4 was shipped into the US in two versions. One, with an intake restriction plate under the carb that reduced it to 12hp and allowed it to be imported under low tariff rates. And a second model, identical except for not having the plate, which produced 18hp and had to be priced higher because of the tariff. So...if there's a useless-looking metal plate under your carb...you might want to check that out with the A4 experts.

[phorvati]

I already removed the restrictor but theres very little difference. Those numbers 11HP and 18HP were only on paper. I can attest that restrictor hardly did anything. Maybe because the entire time I was overproped. I am pretty sure the engine will develop more than 18HP. Just have to get the RPMs up. Transmission is more like a reversing gear box and intergral part of the engine casing, very simple and will not get damaged in neutreal.
The old prop was 18-10 martec folding, bad in reverse. Shaving it off was not an option since I need thrust when under power and in reverse.
Loaded for cruising I think we'll weigh about 17000lbs. I am aware that the little Atomic 4 perhaps has more than it can handle, but I decided to keep it because the thing runs no matter what, very reliable. I took it apart and rebuilt in 2001 and 2003 on my current and previous boat. I added electronic ignition and bunch of other accesories and I feel confident that it can take me to shelter. If needed my wife and I can move it off the stands and into the cabin floor for another rebuild in a matter of hours. I encountered many breakdowns with it and each time I got it running in matter of hours. I have many spare parts.

[hellosailor]

While it is sometimes called the "Anemic4" because it is so weak, IIRC it was also sold as the "Stevedore" engine, i.e. work hard all day every day. They built them with ridiculously low compression, which will mean low power, but feed it, lube it, and yeah, it should run forever--in part due to that same low compression. On a loaded 38....maybe part of the problem is just that it can't develop enough hp to swing "the right" prop at "the right" speed?? What was the original engine on the 38?

[phorvati]

That was the original engine. This boat was configured originally for racing. Since I bought it in 2003 I have converted it into a cruiser. Stevedore was the one with restrictor which I have. I have removed the restrictor. Hopefully I'll noticed more power and more RPMs with a smaller prop.

[mike d.]

phorvati
have you checked with consignment of wickford they are on the s.b. of rt .1 after you come back across the bridge from Newport. you head north on rt. 1 they are only a couple of miles up on left. call them they will give you directions. they had quite a selection when i was there in may. also there is a shop in middletown by Hood/Hinckley. if you go there follow road as if to go to H/H when you go over tracks don't turn in to H/H keep going on perimeter road to next yard. there you will see a marine parts store (new stuff) the store is like West Marine etc. you take left into yard, pull in front. go in tell them you want to go to consignment. they will point the way upstairs it's their shop!!!! GOOD LUCK

regards mke

[mike d.]

phorvati if you only have 18hp you are under powered for theweight. i have GulfSatr 37 and it orig. came from fact. with a 37 hp motor(diesel) the motor was changed to a 50 hp by previous owner and he says he did because the motor was straining before and not enough power in a blow or against foul current. i do not believe you hsve the power to turn a power boat prop. it sounds like you need to change the pitch on your prop. less pitch the higher the rpm. or maybe a smaller diameter to get similar result. but i would stay with a sail boat prop because they are designed to work well (all things being relative) in both directions.
regards mike

[seaclusion]

If your boat weighs in at about 17,000 lbs and your engine delivers 18 hp to the shaft, you are pushing 944 lbs/hp. I don't have my speed/lbs/hp tables handy, but to get anywhere close to theoretical hull speed (1.34 * Sq root WL) you should push no more than 450 - 500 lbs/hp. It's no wonder that you do not get the speed you would like and your engine never reaches WOT RPM.

No matter what prop you put on the shaft, you aren't going any faster than the hp you have now will let you. Of course you could still optimize the prop to a lower speed that will allow the undersized engine to achieve full RPM, but don't expect to get anywhere near hull speed.

Richard

[phorvati]

Then engine curve is 30HP at 3500RPM and 25HP at 3000RPM. At the moment I am running at 1250RPM full throttle which according to the curve is around 10HP and I am going around 5.5kts. I want a smaller prop so that I can run closer to 2000-2500RPM and I am hoping that higher HP will push me faster.

[Jentine]

What is the top RPM that the engine will deliver with no load and in neutral? You have stated that the HP will increase to an acceptable level once you have a smaller prop. That may be true if the engine is capable of attaining that speed at all. The Atomic 4 produces 30 hp at 3500 rpm, but as previously stated, the low compression yields low power and the old girl is most likely tired and will never deliver the yield that you need.
Additionally, you state that your vessel will be loaded at 17000 lbs. By the simplest formula of 500 lbs/horsepower, you either have an excess of weight or a shortage of hp. No prop will give you the panacea that you desire. I would find an acceptable sailboat prop that will allow the engine to accelerate to 3500 rpm and pray that it holds together.