Moving transom - making 10' out of 15' :)

Hello guys,
Was reading forums for some time now, just signed up.

I have an old 15' 77 Glastron T-156 tri-hull.
I want to create a smaller boat like 9 or 10'.
I want to cut it in half on 10' mark and then move old transom or just fiberglass some new plywood transom in. I'll be using 18 HP Nissan 4 stroke for a kicker.
Right now the boat is about 600 lbs and goes 10 MPH cruising and 17! MPH max (somehow?).

I want a smaller boat so my 18 HP will give me 25-35 MPH range..
Don't know how short should I go 8', 9', 10' or 11'.

My inspiration comes from mini-boats of the past (Addictor, Invader etc).

The question I have is how to make the strongest transom - use old with some sides, use old as a cut out, make new.

How to fiberglass transom in, do I need to bolt it, supports or just fiberglass it with bunch of layers - like 3.

The motor is only 114 lbs.

I was also thinking should I leave a little (1"-2") lip around transom to help planning? As of now the boat goes on plane around 10 MPH.

My experience with fiberglass is limited - some small interior work, car pannels.


Will appreciate your comments and advise.

 

A very bad idea, for many reasons. Judging by the basic nature of your questions this is all quite new to you and well beyond the scope of your current abilities, to execute with reasonable safety and success. Buy a 10 foot boat and toss you motor on it. Save the 15' craft for a time when you can swing purchasing a 50 HP or whatever.

 

Thank you for the reply, I kind off expected everybody telling me it's a bad idea. But for this hull it's either junk yard or a size reduction
I don't want to spend $500-$1000 to buy another hull, better get some entertainment out of it and learn some new skills I already fiberglassed one sheet of plywood in the motor well opening to keep high hitting waves on lake Erie out, used just one layer of cloth and did it in few hours at night
Worked fine, so I think I can fiber glass something in. I just need to know how transom ties into the hull..

BTW, looking at how that Glastron was built I was amazed how they used just a little cloth, lots of mat and a few wood/ply structures, If I was building a boat I would use twice as much.

So why is it a bad idea and how it should be done?

 

You'll be cutting the planning surfaces off the boat. You'll alter the hull so radically that it will not float where you expect it to. You will have little more left then a "W" shaped hull, that's too wide for it's length, has it's center of gravity in a poor location, has underwater areas not well suited to planning, etc., etc., etc. . . . Plus a host of other interesting issues, easily destroying the vessels capabilities.

 

The boat has W only in front like first 3', then it becomes V with very wide keel 6" and then 8-12" keel and shallow V.

So I think you can still have some planning surfaces, I saw a pantoon boat plane with enough HP

BTW, on the subject of actually building new transom, any advice?

Current transom is rotted anyway, it was holding 85 HP motor for 25 years, it's visibly bent outward and skin delaminated around motor bolt holes and rusty water spings out when you push on a skin.. so I need to replace transom anyway..

I'm basically trying to turn (left picture) this in to this (right picture of Addictor) -

 

Glastron, Par is giving you some very good advice. Your best bet is to fix the transom in the boat, and then build a 10 footer from scratch. You will learn more, and be way ahead economically. What you are proposing to do is to create a boat that may be completely unsafe to ride in.

I am sure there are plans to build the type of boat you are trying to create. These designs have had years of experience behind them.

Best of luck to you.

 

I have plans for small skiffs that would serve you much better then a butcher job on an old, worn out cathedral hull. Drop me an email if interested.

Glastron156, I've been beating around the bush, trying not to insult your ideas. You clearly don't have enough grasp on the concepts, techniques, methods, physics or dynamics of hull design, engineering or construction to attempt this and then ask people you know and love to come aboard. Small boats are a balancing act for the best designers and a pure crap shoot for those not educated in the necessary disciplines. Small subtle changes in the length, shape, beam or draft can most times be acceptable, but your idea is way out the door and not feasible. No offence meant, just an observation of someone who seems quite determined to do this. If there's one piece of advice you will take to heart, be it this; don't take it father from shore, then you'd care to swim back to . . .

 

Thank you guys for your advice.
I must say - you are very polite, comparing to some forums I been too (Autos)

 

Maybe I miss read but I think PAR was trying very hard to be nice. So are you goung to take PAR up on his offer or butcher the glastron. If it were me, I would take PAR's advice...........seriously

 

No I'm not cutting anything up yet FG will not cure at 32F Meanwhile I'm looking for some project boat on ebay..

Personally, I don't think it's that hard, but you guys have more experience, it's stupid not to listen

BTW, can somebody tell me know, how theoretically transoms attached to hulls?

 

Okay Glastron maybe you have been convinced, heres my next question. Why look for a project boat on ebay, when a designer has offered a set you up with a set of plans that would suite your needs? If you buy a "project boat" it would cost you twice as much as to purchase a set of plans and build from scatch, plus the advantage is, that you would learn a great deal from the experience. If your carpentry experience is little or alot, I'm sure you could find a set of plans to fit the experience you have. And ask alot, I mean alot, of questions.

Just don't go out and cut up a boat hoping it will work and you injure yourself or someone else........Mainly someone else

 

Thanks for the vote Bruce, BTW I bought that 46' Connie, so my interest in your dad's boat will have to wait a little. I still may stop over to take a look, before the show.

Glastron, he's quite right. A fixer up'er will cost a lot more then new (usually around 2:1), not to mention 10' boats are difficult to find. I have a few little skiffs that may be just your ticket to getting wet, cheap. One of them requires a 2 and a half sheets of plywood to build, plus some scraps of 2 by stock for minimal framing. It'll take your little outboard and bring you, a cooler of beer and the fish back safely. Click on my icon and pull up my email to drop me a line.

To directly answer you transom question, most are of sandwich construction. The double flange (tabbing) and skin thicknesses are determined by the loading it must resist and the core used (typically plywood). So technically, it depends on a number of issues, motor weight, thrust, boat weight, target speed, etc. and is application specific. You could look at what remains of the flanges and tabbing and make a guess, but if I remember the boat correctly, they weren't the best built things around. If memory serves, they were copper gun boats with some cloth or roving tossed on, in highly loaded areas. This wasn't the best way to build a hull then or now.

 

Thank you for your answers guys.
Not sure if I have enough spare time to build one from start..

Wife wants me to buy a used jet boat I saw some as small as 12' (usually 14'). They are pretty heavy (1000 lbs) but they do look nice and some people say they handle like water Corvette

 

My small skiff designs could be knocked out in a long weekend, by a reasonably skilled carpenter. There are many skiffs like this available, some being used for boat building contests at boats shows around the country.

The costs surrounding these small craft are generally tied up in the engine, controls and electronics, with a small percentage in the hull, deck and seating.