Under propped 50 horse chrysler on my runabout? Cavitation?

[UglyBoost91]

So another question by a learning newbie ; ) You guys have been awesome for the millionth time, so hopefully you can help me out here too! I have searched the forums but no one seemed to have my exact issues or boat/motor size.

My boat is a 15ft Duo runabout, with a newly power-head swapped 50 horse Chrysler. I struggle to reach 20mph, and feel I should be able to go at least 30 with just me in the thing. I have adjusted the throttle, the engine runs well, 140psi compression, and I have a tach coming this Friday to make sure, but it hears and feels to be @ 5000 rpm.

Soooo, I think it?s the prop. I had a very beat up and bent edge 10.5x12.5, and that is what got me to 20mph, I put on a 10x10 and the boat slowed down but had more accel, which I expected. So my question is, what speed are others doing with a 15ft fiberglass boat and a 50 horse? I want to be able to pull a single small person on a tube to at least 18-20! It is also very difficult for me to make turns, as the engine will rev up and I feel like the prop is just cavitating. Could I be cavitating under load as well? The ?anti cavitation wings? as I call them, are level with the bottom of the boat.

It never feels like the engine struggles at all, I want to just throw a 12x14 prop on this baby and be done with it ; ) hahah

Thanks for the help guys!
Eric

 

[Texasmark]

Re: Under propped 50 horse chrysler on my runabout? Cavitation?

Go to www.go-fast.com and use their prop calculator. Plug in what you know and click on the blank that you don't know, working from left to right initially. Might help as will your tach. I had a 55 Chrysler on a 16' Chrysler sport fury and it would do the 30 with a light load and sorta pull one skier with folks in the boat.

Mark

 

[eavega]

Re: Under propped 50 horse chrysler on my runabout? Cavitation?

You need to know what speed your motor is running. You need to find out if your motor running within its WOT RPM spec, then you can decide what your next step is. Also, you need to have a good prop in order to baseline your setup. A dinged-up, bent prop will most definitely affect your RPMs, so you need to eliminate that factor.
From my experience, I have had a 50 HP on the back of a 15' fiberglass tri-hull, and properly trimmed with a 13P prop I was able to get about 28 MPH with two adults in the boat. It had enough giddy-up to pull a tube, but I don't know if I would've gotten a skier up.

Rgds

 

[emoney]

Re: Under propped 50 horse chrysler on my runabout? Cavitation?

Does the motor sound like it's over-revving a little when you first punch it to get up on plane? Ever feel like it's "slipping" like a transmission does? I'm asking because those are symptoms of a spun hub, and some that deserves to be checked if so.

 

[Maclin]

Re: Under propped 50 horse chrysler on my runabout? Cavitation?

I think any pitch above 12 will lug the engine. I had a 55hp Chrysler on a 15.5 trihull and with an 11 pitch it planed well and would go 29-30. With a 10p the top speed was too slow, with a 12p it could only hit top speed with a very light load. The 11p was perfect for towing and cruising. With a lighter boat the 12p may have worked well.

What trim angle hole are you using in the engine mount? That can make a difference also.

 

[UglyBoost91]

Re: Under propped 50 horse chrysler on my runabout? Cavitation?

Thanks for the quick replys!

Well the 12p rop is preety bad, all the leading edges are bent and look they they have been dragged through a gravel field. In fact the prop is not even 10" dia anymore, more like 9.75". The 10p is a little better, but not much. It does'nt feel like the engine revs up higher on accel, preety linear rpm, but at speed it just feels like it wants to jump forward and haul butt at any moment, but never does. I will have to check on the spun hub.

The motor is trimmed in the second to lowest position out of the 4 holes. It does feel like it cavitates bad, but my girlfriend refuses to drive the boat at full thottle while I lean on the back to look at the prop, something about "safety" ; ) hahah. I think the tach will help also, just being able to know what RPM I am at.

So when I'm searching for a new prop, even Force stuff is hard to find new listed on websites, will evinrude props fit? Reason I ask is most other brands list 10x11 props and such as for 25 and much lower horse motors. Could it be lower gear unit differences?

Thanks!
Eric

 

[Frank Acampora]

Re: Under propped 50 horse chrysler on my runabout? Cavitation?



You would expect to get a top speed of no more than 27 MPH with you alone in the boat. This is based on my Chrysler 55 on a 15 foot glastron. To get this speed I used a 10 3/8 X 13, bronze, three blade Columbian prop. I also did use a 10 X 14 cupped two blade Michigan bronze but it did vibrate and could be felt through the hull.

You do need to experiment with the tilt pin setting for best top speed.

You can still buy these splined, pin drive props for Force 50 engines and a good Michigan aluminum will probably improve your top speed. The stock aluminum props are regressive with rounded leading edges and thick blades which waste power.

There are no add on modifications that can be bolted to the engine for more power and speed--with the possible exception of a retro-fit power trim.

Here are a couple of photos of the engine, a bronze prop, and the 15 foot boat with a much larger engine. If you can get your eyes off the girl, you can see the two blade bronze prop.

Look on ebay for a bronze prop (used) at a reasonable price.

 

[Frank Acampora]

Re: Under propped 50 horse chrysler on my runabout? Cavitation?

Many of the older 10 inch OMC props are the exact same casting with a different hub. However, hubs for these Chrysler and Force engines are getting more difficult to find and are expensive. So, look for Chrysler props on ebay. IF you specify Chrysler/Force propeller in search, you will get about one page of results.

With you and your girlfriend, expect no more than 24-25 MPH tops.

Just noticed in your first post: Anti-cavitation wings? Do you mean a hydro-foil? Try the boat without it and see if top speed improves. The Duo should not need it to either get up on plane or improve handling. I suspect it is actually hurting your performance.

 

[steelespike]

Re: Under propped 50 horse chrysler on my runabout? Cavitation?

Frank knows the Chrysler/Force motors. "Anti cavitation wings" are you refering to the antivent plate just above the prop?
If it has a after market foil on it try it without. Ventilaqtion is what your experiencing.
Cavitation is a different animal. You get yourself a good prop and it should reduce your venting problems.