transom thickness for 40 h.p jet

Hello everyone, Im new as a member to the forum, but have been a fan and refferenced it often for about 3 years. For the first time, I am finding it difficult to find a thread that can point me in the right direction. Appologies if it has already been covered... I am building a project boat with my son. A 14' 6" flat bottom aluminum with a 40 h.p yamaha jet drive. I need advice on the transom as far as thickness, and the suitable degree of angle. I appreciate any input. This forum is an absolute goldmine of boating knowledge.

 

Welcome Mike.

The transom, in way of the jet, can be same as thin as the side plate. This part of the transom is no more than a simple diaphragm to keep the water out.

 

You should clarify the type of jet. Outboard jets use the same transom scantlings as outboard motors.


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It's an outboard..not a waterJET...eeekkk!!...nice catch MP

Why do manufacturers call their bits of kit something it clearly isn't??

My bad...ignore my post above...hahaha

Just make sure you have stiff structure supporting the load bearing parts and it is supported inside the hull. But without more details...hard to give more advice.

 

The OP should consider that a 40hp jet outboard on a small boat is very powerful.

On powerful boats the power of the motor is typically transferred from the transom into the boats structure via longitudinals running the length of the bilge.
I assume that aluminum structural details are very similar to plywood

Perhaps post a picture

 

A 40 HP outboard will need a 1 1/2" thick transom, unless all metal. On an aluminum boat this simply means you'll have your plate outside, with a couple of layers of 3/4" plywood, glued together and bonded to your inner transom skin. You can use all aluminum, which usually involves some square tube stock, for the outboard clamps to smash and a relatively light frame, to transmit loads to the bottom plate. Transom angle should be 15 degrees from vertical, though anything from 12 degrees and up, will do on a this type of jet.

If it's a conventional jet, than you don't need to make any special considerations to the transom (as John previously mentioned).

 

Thanks guys, I should have been more clear that I will be using a 40h.p outboard jetdrive motor. I wont get the motor until the boat is complete, but I have my hopes up for a yamaha, Ive always had such good luck with yamaha bikes in the past. As for answering my question, Par that was just what |I was looking for. I want to shy away from wood and keep it all aluminum on this project.

 

If using aluminum, remember the outboard clamp will place huge compression loads under it, so some compression tubes welded in the bolt and clamp locations, are good ideas.

 

This is the Yamaha Rigging guide. Motor clamp and transom dimensions. Dont know if its valid for a jet.

 

I suspect the same clamp is used, but you should check with Yamaha to be sure.

 

lol what are compression tubes

 

another quick question here. I am going with a flat bottom and intended on having a tunnel for the jet, but somewhere I read not to do a tunnel on a flat bottom, that it can introduce air to the jet. Anyone have some experience with this. I really want to maintain a flat bottom even tho it will be a rough ride. There is a spot on the red deer river Ive been trying to get past on my zodiac for several years and its just to skinny for me. Mind you it is prop job on the zodiac with a 15' shaft, but i can still clear in about 6" of water with only a little prop destruction. The spot is about 4" deep and goes for at least half a block. Maaaaaan I love playing in the rivers.

 

Hi Mike,I'm down here in Calgary and have had a few different type jets.The tunnel won't give you much help, your motor is just to small.You want the water entering the jet to be undisturbed, no air.
Most boats have keels or ribs on the bottom usually with one running down the center.You want this one to stop short of the transom say 30" or so.This keeps the water more laminar as it enters the jet.
20" transom is the norm then a little block /spacer is placed on top to make up the difference so the motor sits right.
I've played with jet hight from level with the bottom to 5/8" down,5/8 down helps with cavatating but you'll get a little spray off of the side of the jet.Flush with bottom most likely will suck air in rough water.
You say 40HP is that at the crank or at the jet?

 

I agree with Tungsten's comments and a compression post or tube is a dedicated piece of tubing, placed just where the mounting bolts and clamp land on the transom. It's mounted perpendicular to the transom, so the bolt or clamp has to crush it straight down it's axis (which is where it's strongest).

Look at it this way; if you have a piece of 1.5" square tube (as an example) and a 1/2" bolt passes through it, with a nut on the other side. As soon as you tighten it up, you start crushing the 1.5" square tube. To prevent this, just drill the square tube and insert a 3/4" length of round tube, welding it to the square tube at each end. Now, the square tube is reinforced right in line with the load and torque the outboard will deliver. You can use square tube to reinforce the area as well, welded in the same way.

 

Wow par that makes a lot of sense. The aluminum will chew away pretty quickly Id imagine with some aggresive boating. Very clever solution, We will do this as well.
Also thank you tungsten, I will stay away from messing with the tunnel in that case and do the ribs as you suggest. The yammy I belive is 40 at the crank and produces more like 30 h.p at the jet. I really wish jets didnt lose so much power compared to props. Its why I never went with a jet on the zodiac. It was rated for 15 h.p and I ran a 25. At 25, in the rapids I was already wishing for more snort. I was told Id be severely gutless if I tried a 25 h.p jet. To expensive to try a gamble on for me anyways.