Do i need to modify my Transom / Get a longer Outboard?

Divertris · 19 May 2012, 18:51

RIB I have used to have Twin 60hp Mariner Bigfoot engines. When I bought it I had to find an engine so I have fitted a Single Mercury 75hp to her.

Today was its 1st outing and a great day it was so much so that I stayed out for 6 hours. However above 1800rpm there was alot of cavitation. On looking at the prop height I suspect I need to lower the engine / Fit a longer shaft engine to clear the hull is this likely to be what was causing the odd cavitation feeling?

Felt like it was revving but no more acceleration and alot of reverberation on backing off it would settle again but disappointing at 20mph.

I will let the photos do the talking:

jezza2011 · 19 May 2012, 19:05

Thats way way too high anti cav plate should be level with the bpttom of the hull. or so i am led to belive.

Boats&Outboards · 19 May 2012, 19:18

Yes too high but I do also remember discussing with you if a 75hp was enough to replace twin 60's ? I think you need at the very least a 90hp to get adequate performance and ideally a 115hp+

Are you re-considering ? might be worth deciding before cutting the transom as a 115hp could one in XL.

Plus you could then fit it correctly, bolted to the transom rather than sat upon it.

Peter @ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

matt h · 19 May 2012, 19:19

your right Jezza, as a general rule it need to level with the bottom of the hull, so drop the engine down till it is!!

thomas · 19 May 2012, 19:29

I was told to have It an inch higher than the bottom of the hull on here

Divertris · 19 May 2012, 19:43

Pete I think the 75 pulls well enough and it was getting up onto the plane prior to the vibration. This one is bolted to the hull? Do you have anything in that would fit well without transom modification? Like to stick with this engine if I can spent a fair amount on servicing & its a good engine.

Open to suggestions on what to do. appreciate any advice.

matt h · 19 May 2012, 20:00

level to an inch is fine, looks like you are 3-4 inches to high!! get your spanners out and get on with it!

Boats&Outboards · 19 May 2012, 20:05

Fair enough. Looks like cutting down the transom is the way ahead in that case. We have nothing better suited right now.

I know it's bolted to the hull but it's entire weight is also resting on the top of the transom, rather than being raised off it and bolted. Obviously not possible with a "long shaft" on that transom as it will be further up again. Your transom must be for a 25" shaft, have you measured it ?

These pictures show some on file fitted without it resting on the transom. See what I mean ?

The last pic is of similar motor, nicked from another Ribnet thread on here discussing the same topic. Have a read of it.

Peter @ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

Divertris · 19 May 2012, 20:08

Quote:

Originally Posted by matt h

level to an inch is fine, looks like you are 3-4 inches to high!! get your spanners out and get on with it!

Spanners wont work here unfortunately keen to modify the transom cutting a slot of the required height if its safe to do so as of course that will be the cheapest option.

Could fit a nice Stainless top to the transom then on refitting the engine for Anti Chainsaw purposes

ribraff · 19 May 2012, 20:09

Hi Tris

1" above the keel is good for this size boat in my experience, this is a good balance of grip in the turns to top end speed. As mentioned that is way too high and looks as though it has been used with a XL shaft, the 75 you've got I'm pretty sure is a long shaft?
If it's set up right it should push your boat along just fine.

Divertris · 19 May 2012, 20:29

Yeah see what you mean peter I assume that is for adjustment purposes having it raised to allow for movement if necessary. Think a cutout is the way forward here then get my stainless man on the case for a nice plate shaped to the transom.

Divertris · 19 May 2012, 20:35

Cheers Edd appreciate your opinion. Do you think cutting a step into the transom is a good option?

Think I will get down there tomorrow and measure how much it needs to drop before i get the powertools out. I will go with lining the cavitation plate 1 inch above the keel.

ribraff · 19 May 2012, 20:42

Chopping the Transom is going to be the only option to get the engine in the right place but you risk wash coming over the back when you are at rest or coming off the plane.

diver 1 · 19 May 2012, 21:21

Tris,

Be cautious of cutting your transom, you'll need to do a neat job and seal it well to prevent water penetration, into the tansom itself but far more importantly, as mentioned by ribraff, you'll then be at risk of swamping the boat when slowing down or with following seas. My first rib was a 5m humber that someone had modified to suit a short shaft engine, the rib was fine and a great laugh with one or two people, but when I put 3 people with dive kit on board as soon as we came off the plane the rib would flood with water almost to the top of the tubes which was dangerous. Even on a choppy day you couldn't sit comfortably and fish because waves would top over the lowered section of the transom.

My current viking tornado is similar. In reverse with the engine trimmed down the water will almost instantly flood over the transom, there is low freeboard where the design of the transom dips to accomodate the engine shaft length, however its better than the humber and actually useful for washing down the deck after fishing or scalloping :-)

Pikey Dave · 19 May 2012, 21:25

Personally I'd get a longer shaft engine. You can't have too much freeboard. Chopping the transom is a copout.

DM · 19 May 2012, 22:03

Liverpool Powerboats have a pair of xl Mariner 90s that they may be converting to long shaft.

Getting the xl mid section off them would solve your problem without chopping the transom.

Divertris · 20 May 2012, 06:05

I did wonder if a section was available to adjust the shaft length. Thanks DM that's awesome if they have one I'll chase it up

Divertris · 20 May 2012, 06:53

Thanks for all the feedback guys. I am keen to keep as much transom as possible so chopping it will be a last resort. Would hate to chop it and regret it due to swamping etc.

Sent Liverpool Powerboats an E-mail and watching a Mid section on E-Bay in the US that may fit. Going to measure mine today and take a few measurements.

Be great to fit another leg and its sorted. Is all thats required is to fit the new section and a longer drive shaft?

C-NUMB · 20 May 2012, 10:41

Longer leg is way to go. But, in a situation where that would be hard/expensive to do,wonder would a transom jack be a possible as an none invasive solution? Like this : CMC Manual Power Lift Transom Jack 65012, and not that hard to sell afterwards if not a workable solution.

Guess these are normally used for lifting the engine so don't know how much You can actually lower it. On the other hand if the engine is further back you can keep it higher anyway.

Divertris · 20 May 2012, 11:38

Transom measured its 23 inches from the top to the bottom of the keel plate