Outboard engine height

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welshhealey
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Outboard engine height

Post by welshhealey »

My outboard engine produces a rather high impressive fountain either side of it whilst going along from 15mph and above, and of course, gets higher the faster you go, I recall sometime back one of our highly respected contributors on this forum suggested the engine is placed too low In the water, :o thing is, I do not want to drill more holes in the transom unless I have to, so here is the question, what is the correct measurements and where from and to what point do you measure from ? and how do I overcome the holes in the wrong place :hmmm: , plus the engine if raised, will have a gap between the top of the transom and the bottom of the mounting frame of the engine, please forgive my ignorance as I'm a novice when it comes to outboards, as all my other classics are inboard .Thanks for the help in advance :thumbsup:

haventaclue
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Re: Outboard engine height

Post by haventaclue »

If spewing water up in a "rooster tail",I would say engine too high or trimmed out too far.
Measure from top of transom to bottom of keel.Measure from bottom of bracket,the arm that hangs on the transom,to cavitation plate.Needs to be within 1 1/2" either side of the transom measurement.Cavitation plate,ideally level with bottom of keel. http://continuouswave.com/whaler/refere ... eight.html
I forgot to mention,holes can be easily filled with an epoxy filler and repainted.
I also forgot to ask,do you have PTT or manual? If PTT trim all the way in until she gets up on plane,then play with the buttons 'til you find the sweet spot.
I manual,put the pin in to,say second hole from bottom,then it's stop start 'til you find the sweet spot.On mine it was the second from bottom on the Merc800.Don't forget to switch off engine while leaning over the transom :D ,and have a string on the pin in case it falls out of your hand :( :(
no matter how bad it is,it can always get worse,I'm an optimist

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welshhealey
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Re: Outboard engine height

Post by welshhealey »

IMG_1467.JPG
It's not the " rooster tail" I may have explained it wrong, if you look at the picture, to the right of the engine cowl you will see a vertical fountain of water ! Yes I've got PTT and engine is toward transom as far as possible, plus the pin you mentioned is in the second hole from the bottom :perfect: when this picture was taken she was planing lovely... speed was approximately 22-25 mph :thumbsup:

Cannot upload image the correct way ... spent ages on it :grrr:
Last edited by welshhealey on Mon Sep 04, 2017 12:27 am, edited 1 time in total.

haventaclue
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Re: Outboard engine height

Post by haventaclue »

It's OK, when I click on the pic it turns the right way up. :D
Check the length of the transom against the leg of your engine,your engine may be too long/short for the boat.Too long,the engine is too deep and water gets past the deflector plate,too short and you have spray and cavitation when turning.
I have been reading that it is a common problem on some Yamahas' because the deflector/splash guard plate is too small.
I have also read somewhere that shape of,or an attachment to the keel,transducer,will cause this as well.
no matter how bad it is,it can always get worse,I'm an optimist

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Rapier
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Re: Outboard engine height

Post by Rapier »

I rigged a Yamaha / Mariner 55hp on a Shetland 570 that had the (anti) cavitation plate below the hull, rather than in line, see pic below. If this is the case, there are a few ways to resolve - raise the motor, redrill new holes; seal the old properly to prevent any damp / water ingress and add a pad under the saddle, or add a more permanent extension to the transom in wood / grp, with a cap (to counter the force at the top of the transom) and lastly use a jack plate (fixed or manually / power adjustable to get the right height).

It depends how bad it is really - my Merc 400 on the Pearly Miss was like that and spewed great volumes of water into the splashwell - I just added the raised pads..the motor was stil attached with transom screws only..never ideal, but I didn't want to drill holes when I know that I wanted to swap motors over. My rule of thumb is never compromise the transom structure unless absolutely necessary...we've all seen rotten transoms. On my Birchwood the fix is too high and given the natural hook they have produces porpoising on the wrong prop & pin setting...older short shafts (non race motors) were 15in transom, Merc produced a 16in shaft length in the late 60s and OMC short shafts are slightly longer and more reliant on greater depth for better cooling..so there is no ideal.
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welshhealey
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Re: Outboard engine height

Post by welshhealey »

Ah right, cheers guys I've certainly learned some valuable info here, one thing I have noticed though, on a sharp turn the prop cavitates, however,I only seem to notice it on a left turn, ( am I right in saying it's like a clutch slipping ? ) Prop seems to loose grip and until I ease off the throttle lever, it does not grip the water again .. I tend to avoid sharp turns now as it's not pleasant when it happens :cry: so, looks like some measuring is in need and the transom modified to get engine to correct height, I would assume once set up correctly, I may get a few mph more ? As there will be less drag off the fountains as seen in the picture :hmmm: I'll just have to get it done pronto before the next rally :o

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Rapier
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Re: Outboard engine height

Post by Rapier »

Measure it up and post here...I think you're experiencing ventilation, not cavitation...with cavitation the prop (if you're using an ali one) will show signs of paint loss around the area where the blades meet the prop hub, so you can normally tell over time. A propeller cup helps the latter, and good stainless props are better at handling the cavitation 'burn' and some even have holes to induce controlled ventilation. What worries me is that it will propably get worse if you raise the engine higher, as it'll be working higher up in all the turbulence caused by a sharp turn. It's also a heavy boat with a smallish engine running a 3in? diameter gearcase - nothing wrong with that but it probably gets overwhelmed by all the air/turbulence caused by the hull. Maybe this is also due to the design of that Shetland hull.
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welshhealey
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Re: Outboard engine height

Post by welshhealey »

Right ok,Rapier, I will go down boat shed soon and take some measurements. Yes the boat is quite a lump and a long shaft Tohatsu 40hp powers it, oddly enough pushes it along quite well all things considered. I will take a close look also at the prop blades. Bearing in mind your concerns about raising the engine, it may be wise to let sleeping dogs lie ? As I'm not likely to do a slalom course with it and can live with the slight " ventilation " so to speak , if your coming to Poole, it maybe a good idea if you come for a spin in it, and with your vast experience with outboards, you may advise me to leave well alone as the water splashing luckily does not fill the splashwell ... not sure about the transom strength either :hmmm: so I don't particularly want to disturb a hornets nest as you can well imagine! :shock:

Diomedea
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Re: Outboard engine height

Post by Diomedea »

I used to have a Shetland with a Merc 50 and put a Doel fin on to reduce this problem.
Seemed better and yes it's ventilation, that is air getting around the prop rather than cavitation which causes the pitting and is the effect of micro implosions in the water causing a shock wave which hits the prop.

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