Transom modification - Long shaft to short shaft

aquaboogie · 03 January 2018, 23:23

I am in need of some expert advise on modifying the transom on my new-to-me 2006 Achilles Se-131. (Catalog here)

I got a great deal on this boat over the summer to replace my old Zodiac when the floor came unglued. After clearing out some other projects I was finally able to get it inflated and mount my trusty 2-stroke Evinrude 35hp short shaft. It looked to be sitting high on the boat and after a quick test run at the bay the prop ventilated at 5kt and I could not get it going any faster. After looking at the catalog I realized this boat was designed for a long shaft outboard.

I began searching for either an affordable long shaft tiller outboard or a new boat to replace this one with. I have not had any luck in that search but I did see a local ad for a similar Achilles boat that had the transom cut and a short shaft outboard mounted. With the deal I got on this inflatable and the fact I really like and trust this old outboard, would it be crazy to attempt a similar modification?

Here is a picture of my Se-131 with the 35hp mounted. Looking at diagrams online for engine height it does not look too bad but my test run proved otherwise.

Here are pictures of the boat in the local for sale ad with a cut transom

My questions are:
1. Should this even be attempted?
2. How much would I need to lower top of the transom?
3. Will these weaken the transom? I am already at the max HP and will use the transom mounted beach wheels occasionally.

beamishken · 03 January 2018, 23:37

The transom should be 15 to 16" from top of transom to lowest point of keel line for a short shaft motor. In some circumstances you can get away with a little longer, personally I'd probably start at 16 1/2 to 17" and see how it performs and be prepared to take it down further if you experience excessive cavitation.
You wont weaken the transom as there will actually less leverage because of the shorter engine. Make sure the cut is sealed with a suitable varnish/sealer to keep the water out and your good to go
Some people will think it's a cardinal sin to cut a transom but if it means you can actually use the boat and engine combo then go for it. No real detriment to the boat

PeterInSa · 04 January 2018, 00:58

I had Merc 45Hp and changed boats, ie Runabout to a 1/2 cabin and the short shaft Merc was unsuitable for the 1/2 Cab. I was able to buy a conversion kit to go from Short to long shaft. But this was some 20 years ago.

Peter

aquaboogie · 04 January 2018, 05:11

beamishken,

Thanks for the info. I was worried about losing transom integrity, but it does make sense that lowering the engine will reduce the leverage it has on the transom.

The whole point of this setup was to keep costs low. I might get started on this project over the weekend.

I just measured the transom height and is 19" form highest to lowest point. I am going to take some more measurements tomorrow and plan the cuts to lower the motor 2.5" for the first shot at it. I'm planning on using a circular saw to get the straight lines started and then moving to a jig saw before finishing it all up with some sand paper.

Any recommendations or advise on cutting and sealing the wood transom?

aquaboogie · 04 January 2018, 06:01

Peter,

I searched for a conversion kit for the Evinrude and there is a 5" extension that does not seem to be in production any more. I would also need 6 bolts(#324334), a new 5" longer driveshaft (#329511), longer shifting rod (#319298), a water pump tube extension (#319340), exhaust seal (#326922, water tube extension (#320025 and it's O-Ring seal (#202893).

It will probably be hard to track it all down and I don't think it would make sense monetarily.

jeffstevens763@g · 04 January 2018, 12:40

https://www.google.co.uk/search?q=in...ui1JtMDkxSe2M:

This probably answers your question go for it as BK says try and keep the cut out to a minimum. You can seal with resin with styrene added to penetrate the wood and top off with epoxy resin to waterproof

Fenlander · 04 January 2018, 19:03

Another vote for cutting down. In the UK most of us have shortshaft motors for SIBs which is what the SIBs are built/designed for.

A short motor will weigh a little less and lower the CofG a bit as well so if you do a neat job I'd not think it will de-value and there will be no downsides.

I just wonder what others think... rather than a cut out might it look better to take the whole transom down across its width as it's what I call a rising transom anyway.

paintman · 04 January 2018, 20:26

The more you cut off the more opportunity for water to get into the boat so I'd vote for the cutout.

jeffstevens763@g · 04 January 2018, 21:03

My boat transom is level with the tube top so as fenlander says it has a rising transom which isn't doing much so the cut out can be at least that depth, personally I would take it down to the depth required making sure you can slew the engine and go with that yes you could get a fair bit of water in if you allow a wave to hit whilst not moving just something to watch, some of the surf cats have open transoms so it won't be a big one in terms of water ingress from a floating point of view but I would fit two decent elefant trunks for quick drainage also look at a neoprene skirt around the engine on the outside to stop water bellowing in.

Mr RIB · 05 January 2018, 19:34

Hi there,

There two ways of curing your cavition issues - you either cut the transom down so that the anti-cavitation plate is half an inch below the transom or you obtain a well cupped stainless steel propeller of the correct pitch and see if that works!

If the latter works, it will significantly improve your boats performance but the stainless prop will hurt your bank balance - cutting a section of your transom away (slight;y wider than your clamp outboard clamp bracket) using a jigsaw is the simple and cheapest option and very straightforward.
Good luck.

Mr RIB · 05 January 2018, 19:36

Hi there,

There two ways of curing your cavition issues - you either cut the transom down so that the anti-cavitation plate is half an inch below the transom or you obtain a well cupped stainless steel propeller of the correct pitch and see if that works!

If the latter works, it will significantly improve your boats performance but the stainless prop will hurt your bank balance - cutting a section of your transom away (slight;y wider than your clamp outboard clamp bracket) using a jigsaw is the simple and cheapest option and very straightforward.
Good luck.

aquaboogie · 05 January 2018, 23:47

Thanks for all of the responses.

I am most likely going to be notching the transom down 3" this weekend and giving the boat a test run. I will cut a notch wide enough for full movement of the motor and a gentle incline up to meet the rest of the transom. It will be similar to the shape of the example boat in my original post. I am not too worried about the increased chance of water coming in from this cutout. This boat will be my spearfishing rig so I will be in dive gear most of the time and it will have a bilge.

I will cover the exposed wood with some plastic and tape while testing and epoxy after I am satisfied with getting the height just right (I found an old thread with tons of good info on this). I have been doing some epoxy research while looking into restoring a well-used speargun and will most likely use the same epoxy for this job. Some trusted gun builders have recommended this epoxy for its moisture protection and durability.

I'll share some pictures of the progress as the boat comes along.

aquaboogie · 09 January 2018, 19:33

So I took the leap and..... Success!

I removed 3" from the transom and the boat performed great on the test run. I hit 23kt with two people and gear in the boat while running in chop. I went for a quick run on my own in a little cleaner water and hit 26kt. I need a new prop and I know these numbers will get better once installed.

I did not experience any issues with water getting close to coming over the transom. I was being really careful due to just having the raw wood covered with tape for testing. I will test further once transom is sealed back up.

Here are some pictures of the cutting process. I will follow up with some finalized pictures after I finish up the work and get it all finished and resealed.

We removed the hardware that would get in the way of the cut and masked off the area we were cutting to protect the finish around the cut. The straight cuts were made with a circular saw

After the straight cuts were done, I followed up the corned cuts with a jigsaw

The end product before mounting any hardware

Old holes filled and hardware re-installed

View from the front

A bad picture of the rear with the motor mounted in its new location. (I will get some better pics when everything is finalized.)

jeffstevens763@g · 10 January 2018, 08:47

that looks like its worked out great the cut out level with the tube top so no issues with water spot on enjoy

Seaflyer02 · 10 January 2018, 19:56

Really nice job. Enjoy.
I used to have the 25hp version on a Avon S400 - happy days!

smokeyjoe47 · 30 January 2018, 12:16

Hi,
I am a newbie to the forum and hope I am not too late for your transom modification. It's definitely a cut out! Measure the height of you your motor ( transom plate to the cavitation plate ), it should be around 14" then have the cut out 5" deep. Take your time cutting it out, be neat and precise, you might want to save the cut out if you ever change back to a long shaft. If the cut out is too low you can always raise the motor by placing placing a piece of wood on the cut out.
Make the cut just wide enough to accommodate a different motor.
Good luck.
Smokeyjoe47

smokeyjoe47 · 30 January 2018, 12:19

Sorry, disregard last thread, did not see lastest posts.

Drulia87 · 30 August 2024, 23:53

Hi,
I’m wanting to do the same thing. Can you send me a clear picture with a straight edge to the bottom v of the transom in correspondence to the cav plate?

What did you end up sealing your transom with. Mine has a rubbery top cap that will definitely be to short after the cut