Prop size on outboard cabin cruiser

[ashedd]

I have a 20' plywood cabin cruiser that weighs about 1200 lbs empty(without motor) and I'm pushing it withal '83 115 hp Evinrude. Prop size is 13.25x17. My tinytach says about 4450 rpm at WOT and gps 28-29 knots. It's gets on plane just fine, I've only ever been on plane by myself with no passengers. I know the v4 omc's arent good on gas but it seems like I'm really chugging gas. Seems to me like I'm lugging the motor. The local prop shop thinks my tach is off, but I think it's fine, shows about 700 idle and 1500 no wake trolling speed. The shop doesn't seem to think I need a flatter prop. What do you think? Im afraid it'll be a real dog with passengers.

 

[jimmbo]

Since your motors recommended rpm range is 4500 - 5500 you are over propped by quite a bit, and you are lugging the engine. A 20 ft boat that weighs only 1200 lbs? Back to your prop, You should be running in the upper end of the range 5000 - 5500. A 17 inch prop on a 115 was most often used on 15 - 17 ft runabouts, so I say it is too big for your 20 footer. Find out if your tach is accurate, I would also be checking things like compression and seeing if the ignition is advancing fully, and that the carbs are open fully. If so then you may have to go with a 13

 

[ashedd]

Thanks for the reply.

I'm going to go out and check to see if the butterflys are opening all the way, and I'll check the link/sync since I never did that. I'm 99% sure the tach is working ok, but I'll see what I can change. My boat is somewhat rare, the only info I can find is for the 18' model and it has a listed dry weight of 800lbs, I still think I'm pretty light. The boat is moored so some of this stuff is hard to do, I have to get my paddle board out to mess with the prop. I looked at props online and the lower pitch ones were being called pontoon props, I thought wow that's not right lol.

 

[ashedd]

The fuel consumption is on par with the 175hp blackmax on my old boat..

 

[steelespike]

A 115 @ wot will use about 16 gph.

 

[steelespike]

I feel that your 17" should produce more speed and rpm.Check what the plugs look like while they are out do a compression check.

 

[ashedd]

I went out and checked the linkage and compared with info I found around the web, I know I should really have a shop manual, and it appears correct. The butterflys open completely, the cam mark hits the center of the roller just as the butterflys start to open. I checked for spark right after I mounted this motor and it checked ok, I replaced a spark pack/module(whatever its called) at that time because of an intermittent spark, well I think anyway. It could very well lose or have weak spark under a load, I dunno.This motor came on another boat I have that I've been working on(18' runabout as a matter of fact). This current boat came with twin merc 500's that needed too much work so I took them off and put this evinrude on there, little did I know the evinrude would take just as much if not more work to get going. Yesterday I was able to eek 4700 rpm of of it with tilting the motor way way up, much more than I thought was ok. I gained a more solid 29 knots and 4700 rpm.

Today I just decided to go buy a cheap prop and got a 14X11. Put that on pretty quickly and went out for a test. First observation is that it vibrates a little at low rpms in the no wake zone, second was that the motor seemed much happier. It didn't sound like it was struggling like before. Once out in the clear I brought it up on plane kinda slowly and at 5500 rpm I was showing 23-24 knots, no vibration, motor again sounded much happier, and I didn't have nearly as much torque steer. The water was a little rough but I was able to go to WOT for a few and was able to get 6100 rpm pretty easy, I didn't note the speed or tilt the motor up anymore but bet I could have gotten a little more rpm's out of it. I settled on an 18-19 knot cruise at 4500 rpm and did that for about 45 mins.

So all in all the boat feels a lot different, more like I think a 20' cabin cruiser should. Before it was feeling more like a speedboat with a door and a place to sleep. I think I need to try a 13 pitch prop and that might be the perfect one. I didn't buy this 11P prop from a prop shop so hopefully I can pull it and pass it off as unused for a return.

 

[ashedd]

I feel that your 17" should produce more speed and rpm.Check what the plugs look like while they are out do a compression check.

I never did a compression check even though I now I should have, even before working on it a few months ago. It runs really well and the video's I shot for another thread got feedback about how well it ran. But I also realize that they can run just fine with low compression or even a dead cylinder.

 

[ashedd]

A 115 @ wot will use about 16 gph.

I was just quoted $9K for a new 90 hp e-tec. Even at 16 gph that would be a lot of gas till I hit 9K. I'm gonna look at a 30' cruiser with a diesel, so that might cure all my problems, besides my affinity for wood boats lol

 

[ashedd]

FYI: I'm under the impression that I should be shooting for a WOT of 5800 rpm for these old v4 omc's, even though the range is like 4500 to 5500. That's what I've read several times anyway

 

[82rude]

Love woodys also.Just not practical for my needs right now so I let this baby go with regret.

 

[jimmbo]

FYI: I'm under the impression that I should be shooting for a WOT of 5800 rpm for these old v4 omc's, even though the range is like 4500 to 5500. That's what I've read several times anyway

It's your engine but I would prop it closer to 5500. the motor makes it peak HP about 5300 - 5400 rpm. Above that and the power starts to fall off.


You say it vibrates at lower speed? What brand of prop is it and does it have a rubber hub, or one of the composite shatter hubs? Sometimes loosening the prop nut and rotating the prop as you retighten it will help it run smoother.

 

[ashedd]

It's your engine but I would prop it closer to 5500. the motor makes it peak HP about 5300 - 5400 rpm. Above that and the power starts to fall off.


You say it vibrates at lower speed? What brand of prop is it and does it have a rubber hub, or one of the composite shatter hubs? Sometimes loosening the prop nut and rotating the prop as you retighten it will help it run smoother.

I don't have a problem aiming for 5500. I'm using a cheapo prop by Turning Point Propellers and their hub which is plastic.

 

[82rude]

Those hubs can also be found in 500 dollar props.

 

[ashedd]

I stumbled upon a used SST prop in a 13P for a good price. Will an SST prop be counter productive? I'm trying to lower the RPM with an aluminum 13P prop, but from what I understand a stainless prop won't lower the rpm as much as an aluminum in the same pitch, correct? Would a 13P SST prop perhaps not lower the RPM as much as I'm expecting an aluminum one to? I think my plan of going with an aluminum 13P prop will just barely bring the rpm down where it should be, at the extreme upper limit.

 

[H20Rat]

A 115 @ wot will use about 16 gph.

Usually 10% of motor HP is the standard, 16gph would be horribly tuned for that engine. Granted, that is older, but the highest fuel consumption I could find in boattest for a 2 stroke 115 was 11 gallons per hour at WOT. Even the 150hp Merc 2 strokes came in just a hair under 15 GPH.

Also, given the year, that MIGHT be a crank rated engine, not propshaft. In that case the 10% number might be high, should be 10 GPH or lower.

 

[jimmbo]

I stumbled upon a used SST prop in a 13P for a good price. Will an SST prop be counter productive? I'm trying to lower the RPM with an aluminum 13P prop, but from what I understand a stainless prop won't lower the rpm as much as an aluminum in the same pitch, correct? Would a 13P SST prop perhaps not lower the RPM as much as I'm expecting an aluminum one to? I think my plan of going with an aluminum 13P prop will just barely bring the rpm down where it should be, at the extreme upper limit.

I've rarely found great differences between Al and SS props from the same manufacturer. The SS usually ran slightly lower rpm due to greater cupping and slightly less slip. That was for the general purpose props. Higher performance props are in a different league

 

[ashedd]

I went ahead and picked up that SST prop, 14 x 13P. It vibrates from about 1000 to 3000, above that it's fine. At 5500 rpm I'm doing 26-27 knots, I'm happy with that. I can push it to 5700 WOT, but gain no more speed, I think that will be fine when loaded with people. Any idea about the vibration? I'm gonna take the stingray fin off and see what happens. Presently I can plane at 15 knots, and fall off at around 12-13.

 

[ashedd]

SST prop is at the prop shop getting an overhaul, one of the blades was tweaked. It's going to be my main prop once finished, I'm using my new backup 15P michigan wheel now. Best part is the prop shop took my original 19P in on trade.

So I think this mystery of prop size has been solved. Hope it helps someone in the future..

Thanks for all the reply's