1957 evinrude fastwin 18hp ventilation problems.

[K.C]

The 1957 evinrude fastwin 18hp I have on my 66 alumacraft maracaibo is having cavitation problems. Can't get motor above quarter throttle before it starts pushing air and losing thrust. I have a 16 inch depth from transom to water line and I believe the motor has a 20 inch shaft. How do I keep it from cavitation when my prop is only 5 inches in the water?? I'm a first time boat owner and don't really know too much of the technical things like prop lengths to transom depths. I do know the boat did not come with this motor in 66 obviously but maybe it's the wrong match for it?
People have told me to notch the transom down a few inches in center but that seems like a last resort thing to do. The motor runs so beautifully and I just restored boat (not to original). I'm in love with it and don't really want to cut it. Any advice would be huge.

 

[flyingscott]

That is a shortshaft motor on a boat that needs a long shaft. Your motor is a 15" shaft and you need a 20".

 

[K.C]

View attachment 343539

Are you sure the prop does not have a spun hub? Try a different prop

I don't believe it does. When me and another person stand at back of boat up to half throttle it's fine. If we walk to front of boat at half throttle it immediately starts feeling like a tranny slipping and I looked at back of boat while going and it doesn't appear to have enough water in the area where prop is. And I can turn the motor over by turning the prop while in gear. It's not slipping. I attached pics to original thread.

 

[flyingscott]

View attachment 343539

I don't believe it does. When me and another person stand at back of boat up to half throttle it's fine. If we walk to front of boat at half throttle it immediately starts feeling like a tranny slipping and I looked at back of boat while going and it doesn't appear to have enough water in the area where prop is. And I can turn the motor over by turning the prop while in gear. It's not slipping. I attached pics to original thread.

Pics came up after I wrote that. I edited my other post.

 

[K.C]

Is that something that I can convert motor to? Or does that mean I can't use this motor on boat?

 

[flyingscott]

Is that something that I can convert motor to? Or does that mean I can't use this motor on boat?

You can find the parts to convert it. I would suggest the AOMCI.

 

[K.C]

I do not believe it is a 15 inch shaft either. Look at my pic with measuring tape. I may be mistaken but that looks to be a 20" shaft.

 

[K.C]

Disregard previous post. I now see that you only measure to where the lower end bolts up.

 

[flyingscott]

Ypu measure from the inside top edge of the hook on the transom clamp to the cavitation plate above the prop. That is where your measurement is. If it was a 20" shaft there would be an extension housing on it.

 

[K.C]

I understand now, thank you for the help. I'm going to try to find parts for it now through omci. Hopefully it's not to big of a job.

 

[flyingscott]

I understand now, thank you for the help. I'm going to try to find parts for it now through omci. Hopefully it's not to big of a job.

It is an easy conversion may even find the parts on ebay. I think the parts from around 58-72 work.

 

[WesNewell]

Either get a long shaft engine or if you want to keep that one get a jack plate for it that will allow it to get down in the water the proper depth. Or redo the transom height. If you notch the transom down, I'd make a splash well for it to avoid taking on water.

 

[K.C]

Either get a long shaft engine or if you want to keep that one get a jack plate for it that will allow it to get down in the water the proper depth. Or redo the transom height. If you notch the transom down, I'd make a splash well for it to avoid taking on water.

It does have a splash well. About 6 inches below transom.

 

[K.C]

I know the "setup" is a bit rudimentary but it's only temporary. You can see splash well here.

 

[K.C]

I've seen people with same problem cut off as little as 2 3/4 inches to make it work but where mine sits I think cutting 4 inches would suit it better.

 

[WesNewell]

You don't want the av plate to be below the hull bottom, so I'd only notch out the minimum amount of transom to accomplish that. If that's 2", then that's all I'd remove. There's no point in removing more than needed. All that will do is weaken the transom.

 

[K.C]

You don't want the av plate to be below the hull bottom, so I'd only notch out the minimum amount of transom to accomplish that. If that's 2", then that's all I'd remove. There's no point in removing more than needed. All that will do is weaken the transom.

Ive watched a lot of videos and from what I can see the cavitation plate or technically the ventilation plate is either level with bottom of the boat or below it a few inches. And every transom depth diagram I look at they say the plate should be at least 3 inches below transom.

 

[Texasmark]

View attachment 343539

I don't believe it does. When me and another person stand at back of boat up to half throttle it's fine. If we walk to front of boat at half throttle it immediately starts feeling like a tranny slipping and I looked at back of boat while going and it doesn't appear to have enough water in the area where prop is. And I can turn the motor over by turning the prop while in gear. It's not slipping. I attached pics to original thread.

Pay attention what Flying Scott said. I agree with him. That flat plate just above the prop should be visible from the front of the boat......get up front with it on the trailer, in the centerline look towards the rear of the boat and start stooping down. At some point you can see clearly all the way to the stern (back of the boat) and see the engine. On yours you can't. Get another boat with a 15" high transom, find a 20" midsection and swap it out....not advised, or get another engine.

I had a '58 18 HP Fastwin when I was a kid....worked all summer earning the $400 I paid for it new. Learned to water ski behind that engine the next season. The flat plate was where I said yours needs to be and I had zero problems with "ventilation" ....sucking air is ventilating.....getting pulled up on skis....considering the engine was only 18 hp.

 

[K.C]

I'm so confused now lol what are the downsides of having the transom cut down besides obvious structural integrity. And my prop is pretty beat up. Could it just be so bad that it is cavitation bad enough to the point of ventilation? I know it's not a spin hub cause I marked the cotter pin and prop and took it out. Marks stayed in line.

 

[jbcurt00]

Seems a shame to hack down the transom on a boat rated for75+hp to accomodate an 18hp motor....