Transom cutting question

[tworotorturbo]

Hi guys, I am about ready to start the heavy work on my project, a 22' Zodiac Hurricane 630. This came with a diesel inboard. I removed the motor and had the old hole in the transom patched up.

I was going to do an outboard bracket but ran into a problem with the bracket company. Now I am about to order one from another fabricator but had a random thought: cut down the transom to fit a 25" shaft outboard.

Is this taboo or totally acceptable? I think structurally it should be fine. Thick transom supported by a pair of large knees. The area will also be reinforced by glassing in a floor straight to the transom. My last ribs had low transoms so I am not worried about water splashing over the top; I actually prefer being able to reverse quickly to flood the deck and wash off the fish blood etc.. That's one of the reasons I thought cutting the transom would be ideal. The other reason is to keep the overall length down.

The original water line at rest, with the old 450+ kg inboard setup, is where the blue line is drawn. The black lines show what it would resemble if I cut out an area for the outboard. Thanks fellow ribbers!

[Pikey Dave]

Personally, & this is purely my opinion, I'd avoid cutting the transom like the plague. I don't think strength would be an issue, unless you cut into the "patched" area where the outdrive penetrated the transom. I think you will end up with a very wet boat, also any outboard engine sat there will be very low in the water, the water will be lapping under the powerhead.


Just my 2 penneth.

[Last Tango]

Difficult to say without actual dimensions but TBH doesn't look too bad. If I were going for this option though, I'd seriously consider fabricating an engine well to restore the freeboard integrity.

[Searider]

I think you'll be fine - subject to the repair of the sterndrive aperture being good.

My boat only has about 6" freeboard at the outboard.

I would also think that the weight in the stern will be much less than the 450kg engine you've removed. Heaviest outboard is about 300kg?

What's the distance between your blue and black lines?

[cgoing]

You should try searching previous threads for Hurricane 630 and 640 builds. I may be mistaken but I believe a member on here had cut down a H640 transom as part of an inboards to outboard swap. Keep in mind if you do cut it , it has to be wide enough to allow the motor to tilt side to side when in the raised position.
I am still undecided on my 640. Probably going with a jack plate rather than a bracket. The bracket would push my length into a larger dock slip which significantly would raise my annual dock slip fee.

Good luck and keep us posted!

[Maximus]

Originally Posted by Pikey Dave

Personally, & this is purely my opinion, I'd avoid cutting the transom like the plague. I don't think strength would be an issue, unless you cut into the "patched" area where the outdrive penetrated the transom. I think you will end up with a very wet boat, also any outboard engine sat there will be very low in the water, the water will be lapping under the powerhead.


Just my 2 penneth.

I agree with PD!...on the pics that's a hell of a lot of material to loose from one of the most structurally important part of you're boat!..and could open a can of worms from potential transom failure,to re-sale implications.
It can be done on certain models,but really do you're home work and double check with people who have done the exact same modifications your on about.
(See below)

[whisper]

You wouldn't cut the transom, you'd add an engine bracket off the back?

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[cgoing]

This is a factory transom height on an outboard version. Much lower than tworotors I/O version.

[Pikey Dave]

That looks very low, my transom is around 3/4 way up the tubes.

[Poly]

Originally Posted by cgoing

Attachment 110468

This is a factory transom height on an outboard version. Much lower than tworotors I/O version.

that looks like it has a "bulkhead" a few feet in front of the transom though?

[Pikey Dave]

Originally Posted by Poly

that looks like it has a "bulkhead" a few feet in front of the transom though?

I'd missed that👍 which effectively makes the transom a built in bracket

[The Gurnard]

Originally Posted by Pikey Dave

I'd missed that�� which effectively makes the transom a built in bracket

Nope..it makes it a cut down transom with an internal well to keep out the swell

Is that splitting hairs ?

[Pikey Dave]

Originally Posted by The Gurnard

Is that splitting hairs ?

Yup 😄

[cgoing]

Ok here is a photo of the sister ship to the red one I posted. Yes there is a raised area in front of it with a drain system to allow water to drain out. The water line is below the three holes between where the outboards were removed. Plenty of outboard out of the water. These hulls float very high .

[tworotorturbo]

Originally Posted by Searider

I think you'll be fine - subject to the repair of the sterndrive aperture being good.

My boat only has about 6" freeboard at the outboard.

I would also think that the weight in the stern will be much less than the 450kg engine you've removed. Heaviest outboard is about 300kg?

What's the distance between your blue and black lines?

Thanks for the feedback guys. Distance is 9.5". My SR6 and SRMN600 had even less freeboard and I did not ever see it as a problem. I have a Honda 150 ready to go on it now but may do a Yam 250, so heaviest OB on there will be less than 300kg for sure.

Here are a few more pics. One thing I need to find out is if the motor can be fully turned on either side when trimmed up (thanks for reminding me cgoing). The interior shot shows the knees. I would make a splashwell between the knees after I put in a floor. The black lines show what the cut would look like.

[Last Tango]

I'd imagine the fixed trim tabs might be superfluous too ?

[Chris]

What about getting an Ultra Long Shaft on there... would reduce the need to have such a deep cutout.

[9D280]

Chris, you beat me to it!

That looks well low to the water. I wonder - will the idle ports on your exhaust even be above the brine?

Next silly Q - have you already got an engine? If not - hows about twins? - they'll sit further out and so not need to go so low to clear the hull.....

[Last Tango]

Originally Posted by 9D280

Chris, you beat me to it!

That looks well low to the water. I wonder - will the idle ports on your exhaust even be above the brine?

9.5 inches is a pretty fair clearance. Mines (all factory spec) is a lot less than that and there's no issue with the exhaust idle ports ports. (see the picture on my profile). The engine-well preserves the integrity of the freeboard and that's where you are going to get into trouble. I've had a good few boats where the ports are just above the water and it's never caused a problem.

[60knot]

I would be careful cutting away the transom to close to the knees. The close change in section will weaken the top of the knees close to the cut out.