Underpowered or overpropped

I have a 1978 Wellcraft Airslot 165, it's just under 16.5 ft. My boat has never been too fast, it'll go about 33 on glass. I'm not too concerned about top speed as I cruise at about 3500 rpm, I don't like running at full throttle. Anyway I have a 1983 chrysler 90 4 cylinder. It had a 17" prop and the rpms would get up to 4500 but I want to get up to 5000 at WOT. My RPM range is 4250-5250 mind you most other 4 cyl chryslers are 4500-5500 so I'm confused about that but my ID plate says 4250-5250. Anyway my old prop god "shaved" down pretty good so I bought 13.5x15" thinking a drop in pitch would get my rpms up but it didn't?? They fell a little, lower than I'd like. The prop looks pretty wide so mabye that's the problem but with the drop in pitch you'd think it would offset it at a minimum, not cause a drop in rpms. Now, I THINK my boat is heavy for a 16ft, one site says it weighs 1650lbs so mabye it's a TAD bid underpowered?? Opinions please

Re: Underpowered or overpropped



First a little history:

Chrysler always made a 90 but the original engines were 3 cylinder. They were rated at the crankshaft, not the prop. Chrysler also always made a 4 cylinder 105. It was likewise rated at the crankshaft. These engines derived their horsepower rating from a complex formula that included the bore and stroke.

Around 1982, the entire marine industry went to propshaft rating. Chrysler downrated and rebadged the 105 as a 90. Even so, it was a weak 90 compared to OMC and Merc.

One of the ways that Chrysler controlled the horsepower rating was to rat different engines at lower or higher RPM. THUS: if you get your 90 up to 5000 or even 5500 RPM it will deliver a more honest 90 HP. Don't worry, Many of the 4 cylinder engines were rated at 5500 and they are all the same block, so, you won't kill it.

NOW: I think your 90 had the small 1 5/16 venturi WB carbs. You can bring it up to 90 simply by finding a set of manifolds and TC carbs off any Chrysler or Force 4 cylinder engine--120, 125, 135, 140.

SO: are you underpowered? Perhaps a tad. Overpropped? Perhaps so. Possibly both.

It sounds like you bought a prop with a LOT of blade area designed for moving displacement hulls. You would probably do better with a stock type prop AND, stainless will give you about 200 RPM more.

A couple of photos of the carbs and prop would really be appreciated. After all, I showed you mine! LOL

Re: Underpowered or overpropped



Couple more photos

Last two are my 105/90 It drives this 15 foot Glastron at 45. Now I doubt that you boat will be able to get that high (Glastron only weighs 700 bare) but you should be able to get high 30s.

Re: Underpowered or overpropped



Frank you could at least make me work for it before showing me lol!

A few pics here, look at the one on land first to see how much boat is "in the water". Like I said, I think it's a heavy boat for a 16.5ft and has a lot of hull in the water. I'd like a 5000 max, I don't want WOT to be at it's absolute red line! I'll definately try the carb and manifold thing once I get some extra money. It's a 13.5x15 like I said but going by the app guide here the 15" pitch is 13 1/8 so that's 3/8 narrower than what I have on it. Time being can I have the prop width taken down?? I know a prop repair place not too far from me I don't know if this is something they do to props though??

Re: Underpowered or overpropped

healey8390 said:

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Frank you could at least make me work for it before showing me lol!

A few pics here, look at the one on land first to see how much boat is "in the water". Like I said, I think it's a heavy boat for a 16.5ft and has a lot of hull in the water. I'd like a 5000 max, I don't want WOT to be at it's absolute red line! I'll definately try the carb and manifold thing once I get some extra money. It's a 13.5x15 like I said but going by the app guide here the 15" pitch is 13 1/8 so that's 3/8 narrower than what I have on it. Time being can I have the prop width taken down?? I know a prop repair place not too far from me I don't know if this is something they do to props though??

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I hope I didn't confuse anyone with the look at it on land to see how much is in the water lol. I was trying to say it's got a pretty deep hull and you can see how much is in the water if you compare the two pics.

Re: Underpowered or overpropped

Yes, that looks like a heavy hull. It does remind me of the old Chrysler boats with the small sponsons in the front--not really a cathedral hull.

That prop looks like the Vortex prop I have. I don't use it because in the same diameter and pitch, it does load down my engines way more than the stock type props and does not deliver more speed.

While a good prop shop can reduce area and change pitch, I would not mess with that. You don't know what the final performance result will be and you have invested money. Besides, When my daughter did not listen to me and "found " rocks, I did cut down an 18 spline, two piece gearcase, Michigan aluminum by about 3/4 inch. Essentially, it performed about the same, maybe 200 RPM more--I don't remember exactly.

If you go onto ebay and search for Chrysler and Force propellers, you will find a couple of 17 pitch old (stock) props (that you can not find here at iboats). Some are a bit expensive and some are reasonable. There are even one or two stainless for under 200 bucks.

Let me disabuse you of the notion that 5500 is the "redline." It is just the manufacturers upper performance limit for recreational use. Note that my 125 was run at 5900 on my little 14 foot go-fast. Note also that some outboard manufacturers list 6000 as their upper range limit. In fact, since there are no valves to "float" These two cycle engines can exceed 6000 by a wide margin without damaging themselves.

Of course, the higher the RPM and horsepower output, the shorter the engine service life. An extreme example would be a 10,000 RPM, 5000 HP dragster whose engine life is measured in seconds--they are rebuilt after every 1/4 mile run.

However, running your moderate horsepower engine at 5500 or slightly more will do nothing to decrease its service life.

I can not tell from the photos, but if your cavitation plate is sitting even with or slightly below the hull bottom, you MAY benefit by raising it to an inch above. This does have performance gains on some hulls but does result in ventilation on others. You don't know unless you try.

My cuddy, for example, WILL ventilate and cavitate with the engine that high. In fact, even now with the plate even with the bottom, I can only use about 1/2 of my available trim. More than that and she loses grip. Yet, the boat in the avatar has the engine raised and it is solid, even at full trim, and even in turns.

Re: Underpowered or overpropped

I found a good deal on a pair of force 125 carbs with the minfolds and everything for $120! I don't like the WB carbs anyway. The fuel bowl screw is the main jet and I learned that the hard way a long time ago as I twisted the head off when I tightened the bowl after servicing the carbs.

I called a local prop shop and told them of some props I've been eyeing. We came to the conclusion of going with a 14x11 and we can go from there to modify it if needed. I'm hoping with the bigger carbs and the 14x11 I can get to at least 5000 so we'll see...

Re: Underpowered or overpropped

I'm also hoping being in the right range will improve fuel economy. My "logic" tells me that if I currently max out at 4200 and crusie at 3500, that's already pretty close to its limit. So if I get my max to 5000, the 3500 will be an easier 3500 and be better on gas. Any thoughts on this??

Re: Underpowered or overpropped

healey8390 said:

I'm also hoping being in the right range will improve fuel economy. My "logic" tells me that if I currently max out at 4200 and crusie at 3500, that's already pretty close to its limit. So if I get my max to 5000, the 3500 will be an easier 3500 and be better on gas. Any thoughts on this??

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i went from a 17 to a 15 pitch and it seems to be little easier on gas b/c my max rpm went from 4500 to 5100 and it seems to be more at ease.

Re: Underpowered or overpropped

Looked at the pic of your present prop.It looks like an aftermarket or a Yamaha prop.
Get the factory prop. Michigan wheel sells a good prop.
A 14 inch might be too big.
Trying to make the wrong prop work will get you results, usually bad.
They tell you that 5200 is max. Then 5200 is what you should shoot for.
Lugging at 4000 can do damage.The engine needs to wind up for best performance.

A friend asked me to check out his boat as it wouldn't run as fast as it should.
I checked EVERYTHING!!! It all checked out ok.
I then noticed an old prop setting on a bench. Turns out they had just changed to an aftermarket prop.
Same pitch,different diameter. Put old prop back on and problem soved..This was on a Volvo I/O.

Re: Underpowered or overpropped




NO! NO! NO! While a 14 inch diameter may fit on the engine--in fact there is just over 7 inches from center of prop shaft to ventilation plate--- 11 pitch is just wrong. While you do have a heavy boat, it is not a pontoon or displacement hull for which that prop is designed. You will find that with the 11 pitch rpm will go up but speed will go way down. Don't even consider using it.

AND go to another prop shop that knows what they are doing. That's just bad advice.

In your application, the correct prop will be somewhere around 13 to 13 1/2 inch diameter with pitch being somewhere between 13 and 17 inches. Like Jerry said: Go back to a stock prop. In your case, the closed hub aluminum or stainless props that Michigan made for Chrysler. (bottom prop in the center photo.) There are a couple on ebay now--Did I just repeat myself? I hate when I need to do that. LOL

Re: Underpowered or overpropped







The first 2 pics,the right prop is a Force prop.
Last pic is what they look like on the motor.
You have the exhaust snout???

Re: Underpowered or overpropped

Frank Acampora said:

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NO! NO! NO! While a 14 inch diameter may fit on the engine--in fact there is just over 7 inches from center of prop shaft to ventilation plate--- 11 pitch is just wrong. While you do have a heavy boat, it is not a pontoon or displacement hull for which that prop is designed. You will find that with the 11 pitch rpm will go up but speed will go way down. Don't even consider using it.

AND go to another prop shop that knows what they are doing. That's just bad advice.

In your application, the correct prop will be somewhere around 13 to 13 1/2 inch diameter with pitch being somewhere between 13 and 17 inches. Like Jerry said: Go back to a stock prop. In your case, the closed hub aluminum or stainless props that Michigan made for Chrysler. (bottom prop in the center photo.) There are a couple on ebay now--Did I just repeat myself? I hate when I need to do that. LOL

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Hey Frank... I bought the gaskets for the bigger carbs and manifolds today. I asked his opinion and while he agreed that it's a heavy boat for a 16ft he said somethings not adding up. He said with a 15 pitch I should be 5000-5500... He asked me if it's misfiring and if I'm getting any fuel sheens behind the motor. I told him it runs great, will throw the boat onto plane instantly. He told me the tach could be off so investigate that further before doing anything to crazy with the props... He said use an inducitve tach that goes to a plug wire as they are really accurate. Hell he even told me to tell him when I'm doing it and he'll dig his up and loan it to me!

Trust me I didn't want to go with an 11 as that sounds rediculous for a 16ft boat with a 4cyl outboard??? I just want the engine within it's performance range TBS the bad news is I alreay have the 15 for sale on ebay and the auction ends in 45min LOL!!!