Prop help for older boat 15' trihull w/50 HP Evinrude

[eavega]

I have finally put my restoration project on the water, and had a qualified success, but I had a few questions, specifically prop related.

The Motor - 1972 Evinrude 50 HP model 90273C.

The Prop - Assume it was the original that came with the motor. Its a 13 1/4" 17 Pitch aluminum prop (that's what is stamped on the propeller).

The boat - 1975 Yarcraft Yankee 15' trihull bowrider. Per the research I have been able to do, it weighs 800 Lbs dry. Now this may be inaccurate, because I suspect this boat had a new deck and transom put on it in the last 35 years, which may have changed the weight. The hull is rated for 70 HP max, so I understand I am starting a little underpowered to begin with.

A typical load for this boat would be about 750 Lbs (3 fishermen, cooler, trolling motor, extra battery, fishing gear).

I am assuming the tach is accurate, but it is not original to the boat, and I don't have any experience with it. It is correctly connected to the pulse wire coming out of the controls. Verifying the RPMs would be awesome, but I am not sure how, short of buying another tach and seeing if I am getting the same readings.

The issue - Its giving me 4400 RPM at WOT. The speed was fine at about 20 MPH (I wouldn't mind more, but I'm okay with that speed), and it does plane fairly quickly (didn't jump out of the water, but it got to where it needed to be in short order), but I don't want to be lugging the motor or stressing it by not allowing it to run at its recommended 5500-6500 RPMs.

Is the proper course of action to throw on a 15p propeller to raise the RPMs? What is the implication in terms of speed? Or, am I just better off leaving the 17p prop on there and running the motor at a lower speed? Other considerations?

Thanks in advance for your help.

Rgds

Eric

 

[steelespike]

Re: Prop help for older boat 15' trihull w/50 HP Evinrude

Assuming your test was with a light load your way off.You need 1100 rpm just to
get to the bottom of your rpm range. That would put you at about 12" thats pontoon prop area.Is speed by gps?Does it respond to trim up.Check that the carb is opening all the way,that the stator is advancing as it should.firing on all cylinders,good compression,
Your antivent (cavitation) plate just above the prop should be about 1" above the bottom of the boat.Prop in excellent shape.Bottom clean and smooth.

 

[eavega]

Re: Prop help for older boat 15' trihull w/50 HP Evinrude

Haven't been able to work on the engine any, but I did confirm that the GPS might be off on its speed readings. I took it with me on my morning drive, and it was consistently reading about 5-6 MPH lower than it should have based on my car speedo, which is fairly accurate based on those mobile radar with displays that the police put out to warn drivers when they are breaking the speed limit.

So, the speed should be in the 25 MPH range. I definitely need to address some issues on the motor, as it would not run at idle. Definitely a carb adjustment is in order, perhaps a cleaning. Additionally, the only verification I made on the water was that the carb butterflys were fully opening and closing though the travel on the throttle. I had verified spark, but not the timing. Since it was not running rough once it got up to a happy speed, I didn't think it a priority. The idle performance is definitely a concern, and my first area to address.

I have also ordered an optical test tach to confirm my console tach is accurate. I am trying to eliminate any inaccuracies with my diagnostic tools before I start messing with the motor itself.

In any case, I am wondering if I am, in fact, still overpropped with a 17p prop. I am still feeling that I am somewhat underpowered with a 50 HP running on a boat rated for a 70 HP motor. My concern is really to get the motor running in its correct range, not so much getting more speed. Running through an online calculator, it seems that if I step down to a 13p prop and can get it running at 6000 RPM, I would gain a little speed versus lugging the motor at 4400 RPM WOT with the 17p prop (assumption is that once I get the motor correctly tuned, I should be able to get the RPMs up close to the 5000 RPM range)

-E

 

[bekosh]

Re: Prop help for older boat 15' trihull w/50 HP Evinrude

In any case, I am wondering if I am, in fact, still overpropped with a 17p prop. I am still feeling that I am somewhat underpowered with a 50 HP running on a boat rated for a 70 HP motor. My concern is really to get the motor running in its correct range, not so much getting more speed. Running through an online calculator, it seems that if I step down to a 13p prop and can get it running at 6000 RPM, I would gain a little speed versus lugging the motor at 4400 RPM WOT with the 17p prop (assumption is that once I get the motor correctly tuned, I should be able to get the RPMs up close to the 5000 RPM range)

-E

I used to have a very similar setup. 1972 Sea Star 15ft Trihull with a 1984 50hp Mercury. Same as yours it was rated for 70hp.

I ran a stainless 13p prop and maxed out at 28mph. Don't know what the RPM's were because I didn't have a tach. With the aluminum 13p that came with it I ran about 26mph. All speeds are GPS.

So I would say that you would do better with a lower pitch.

 

[eavega]

Re: Prop help for older boat 15' trihull w/50 HP Evinrude

Just as a follow-up to my original post. I did finally get a Solas Aluminum 13p prop. I finally got to put it on the boat this weekend. These were my observations;
I had the boat loaded with myself (200 Lbs), my 2 sisters (about 120 Lbs each), and their two sons (about 50 Lbs each), so estimate total passenger weight at 540 Lbs. Add to that 12 gals of fuel, battery, and cooler, and I was probably beginning to approach the safe carrying capacity of my boat. Ran it out from the dock, and couldn't get the motor to rev above 4K RPM this time. I was starting to fear something really wrong with my motor. I took the folks back to the dock in preparation to load the boat back onto the trailer take it home to start tearing into the engine. As I started to move the boat from the courtesy dock, I felt it really jump a lot more than it did previously, which made me curious. I got out of the no-wake zone and opened it up. YeeeeHAWWW!! boat comes up on plane, running about 25 MPH (about what was expected given the boat's weight, HP, and prop charactaristics), and more importantly, the motor is now turning 6K RPM! Apparently when you don't overload the boat, it will turn as its supposed to. I then got each of my sisters on the boat, one at a time, and learned that the boat will still get up to about 5.8K RPM with a passenger weight of 400 Lbs (me, sister, one kid, and the cooler).

It is a dog until it gets up on plane, no hole shot at all. For what I want to use this boat, though, it is acceptable. I may try it again with the 17p prop totally unloaded to see how it performs, but for now I think the most logical conclusion is that the 50 HP motor is a hair underpowered for this hull's maximum rated weight. For two regular-sized guys, fishing gear, and a small cooler, though, it will be totally acceptable.

-Eric