the thrust from an inboard on a 26 foot yacht

[dylanwinter]

Now that i have Wessex on board with jollop. advice and bloke to to the resin work

I am reasonably confident that the well will be watertight and all other orifaces made watertight as well

and I am assuming that Tohatsu think that their ultra long shaft has a good chance of being the right engine for the job

otherwise they would not have agreed to help with the project

I just wanted to know about the way the thrust from the engine is transmitted to the hull in a small bilge keel yacht - say 26 feet or there-abouts



I assume I have to come up with a design that will take the engine thrust back to the engine mounts

but where else does the thrust go

especially if I am going to try to keep 45 degrees of vectoring

Dylan

just a reminder....

here are the pictures of the back of a Centaur

https://plus.google.com/photos/110182886418433827802/albums/5937688592940855473?banner=pwa

 

[prv]

All thrust through the engine mounts in most cases, unless you have something like the Halyard anti-vibration drive which incorporates a thrust bearing. Kindred Spirit's Yanmar used to scooch forward half an inch as it went into gear.

Pete

 

[AngusMcDoon]

I reckon that the thrust from a 10hp is so minimal that as long as it is well attached to solid hull then that will do. It's not as though you are installing a couple of 500hp Mans.

 

[alahol2]

If the engine were on the transom it wouldn't send thrust anywhere near the engine bearers so there is no need for your design to send it there either. It will be the fixings of the well itself that will transfer the thrust to the hull/superstructure.

 

[dylanwinter]

All thrust through the engine mounts in most cases, unless you have something like the Halyard anti-vibration drive which incorporates a thrust bearing. Kindred Spirit's Yanmar used to scooch forward half an inch as it went into gear.

Pete

Thanks Pete,

I am hoping that most of the well will be made from 1 inch marine play

is that man enough?

I can double it up at the stress points

anyone got any design tips for getting the thrust back to the engine mounts

I can over-engineer because marine ply is relatively cheap and the epoxy is gratis

On Katie L the thrust is taken by a massivly strong tunnel under the boat

all suggestions welcome

 

[dylanwinter]

Thrust

I reckon that the thrust from a 10hp is so minimal that as long as it is well attached to solid hull then that will do. It's not as though you are installing a couple of 500hp Mans.

I hear what you say

but I was surprised at the forces involved when we did the static pull tests at the sailing club for the 6hp outboards

pretty big forces - and this is almost double that

I don't want the well to flex around and break the lovely waterproof joints

I have two engine mounts - so I would be a fool not to use them

not sure what happens when in revese or vectored

I assume the torque on the top clamp with an ultra-long shaft must be horrible

in forwards you can share the loads around a bit more

Dylan

 

[Tranona]

You are making it too complicated. You don't have to consider the existing engine bearers. You will need a transverse bulkhead glassed into the hull the full width and up the sides. 1" ply would be more than enough. Have a look at an installation of a Halyard Aquadrive which does take the full thrust. It is usually a ply web around 15-18mm thick glassed in. Use epoxy and glass cloth. Wessex will know how to do it.

Despite the engine producing nominally 10hp, thrust will probably be pretty low compared with a conventional prop driven by a diesel and a 1" board if properly bonded in will be more than adequate. Don't think you will be able to do much vectoring as there is unlikely to be enough room to turn the powerhead more than a few degrees. Remember also your prop will be right in front of the rudder so in forwards you will get plenty of propwash and the boat will probably turn in its own length. Be prepared for dismal performance in reverse, both poor stopping power and no control over where it goes. A standard inboard Centaur is not at its best in reverse and your outboard powered one will be even more pathetic.

 

[ghostlymoron]

I'm surprised that Tohatsu have agreed to have their name assiciated with such a crack pot scheme. It could make them look foolish.
I was jolly glad to get rid of the outboard and get a properly engined boat.

 

[prv]

Have a look at an installation of a Halyard Aquadrive which does take the full thrust. It is usually a ply web around 15-18mm thick glassed in.

+1

Halyard make drives suitable for quite large numbers of horsepower and, as Tranona says, they take the full thrust of the prop because otherwise the short shaft between the universal joints would jack-knife. The instructions for the Halyard drive simply tell you to glass a low bulkhead into the bottom of the hull.

I think any construction that is strong enough to be safe against flexing and leaking will be strong enough to clamp the engine to.

Pete

 

[seadog30]

Hi

One inch ply encapsulated in GRP should not be a problem with constant loads. I sugest that you consider 2 things in addition to the general load.

1) shock loads i.e comming down off a wave and the prop bitting again.

2) vibration, make sure that the critical frequency is not at the same level as the engine revs. I would break up any flat areas with moulded in ribs both in the new ply bulkhead and any any surrounding GRP areas.

Good luck

 

[Uricanejack]

What you need is a "thrust block".
you might be able to get on shipped from breakers in India. They only weigh a few tons.

As said above you are over thinking.
what you need is a firm mounting for you outboard which will not move through the boat.
So when the propellor or screw turns the boat moves.

A thick piece of half inch ply covered in resin so it will not get wet will do the trick. An internal transom at the forward end of your well.
Bolts through the ply will be better than the screw fittings which your turn up by hand.

The interesting bit will be determining the vertical position.

Still wonder why you have your heart set on a well. A bracket on the existing transom would work just fine.

I used to sail a Cal 20 with a 5hp Mariner outboard in a well. there was a piece of hard wood at forward end of the well raised above the cockpit sole over which the mounting bracket was placed and the hand screws tightened on.
The waterline on the cal 20 was just a few inches bellow the cockpit sole.
The question with a 26ft centaur is how far bellow the cockpit sole is the waterline.?
Can you fit the nessesary piece of hard wood or ply above the sole and have the long shaft extend far enough to get the screw bellow the hull and deep enough for the cooling water.?
or do you need to have the hard wood bellow the sole with the bolts on bracket through the wood turned up from inside. which defeates at least one possible advantage of your outboard. access.

good luck with it.

 

[dylanwinter]

What you need is a "thrust block".
you might be able to get on shipped from breakers in India. They only weigh a few tons.

As said above you are over thinking.
what you need is a firm mounting for you outboard which will not move through the boat.
So when the propellor or screw turns the boat moves.

A thick piece of half inch ply covered in resin so it will not get wet will do the trick. An internal transom at the forward end of your well.
Bolts through the ply will be better than the screw fittings which your turn up by hand.

The interesting bit will be determining the vertical position.

Still wonder why you have your heart set on a well. A bracket on the existing transom would work just fine.

I used to sail a Cal 20 with a 5hp Mariner outboard in a well. there was a piece of hard wood at forward end of the well raised above the cockpit sole over which the mounting bracket was placed and the hand screws tightened on.
The waterline on the cal 20 was just a few inches bellow the cockpit sole.
The question with a 26ft centaur is how far bellow the cockpit sole is the waterline.?
Can you fit the nessesary piece of hard wood or ply above the sole and have the long shaft extend far enough to get the screw bellow the hull and deep enough for the cooling water.?
or do you need to have the hard wood bellow the sole with the bolts on bracket through the wood turned up from inside. which defeates at least one possible advantage of your outboard. access.

good luck with it.

I am hoping... still hoping to keep it simple by having access to the screws above the level of the cockpit but until Roger measures the distance for me I am still unsure

taking on Cape Wrath with an outboard on a bracket and all those nasty bars with their short chops is not something I fancy
I have just edited a film of me messing with Wells bar

it was bloody horrible with an outboard on the back of the slug

I then turned around and did the same thing using the slug inboard

it was that experieent - plus my experiences with eboats and sonatas that has set me on the well course

D

 

[Applescruffs]

It's not just the thrust to think about.

You've also got the rocking motion of the boat , fore and aft and side to side.

I'm pretty sure that a reasonably heavy engine anchored to the top of your well will impose all sorts of stresses around the hull / well joint when it's rocking about......leverage action....fulcrums....that kind of thing...a bit like repeatedly bending a piece of plastic until it breaks .

I reckon that the whole thing should be braced at the top to the cockpit sides and anywhere else you can, it won't look pretty though.

One other point....is winter really the best time to be working outdoors with fiberglass and resin etc ???

Anyway...all the best with the project, and I'll be the first to raise a glass of the 'ole Janx Spirt when it's done......

 

[dylanwinter]

It's not just the thrust to think about.

You've also got the rocking motion of the boat , fore and aft and side to side.

I'm pretty sure that a reasonably heavy engine anchored to the top of your well will impose all sorts of stresses around the hull / well joint when it's rocking about......leverage action....fulcrums....that kind of thing...a bit like repeatedly bending a piece of plastic until it breaks .

I reckon that the whole thing should be braced at the top to the cockpit sides and anywhere else you can, it won't look pretty though.

One other point....is winter really the best time to be working outdoors with fiberglass and resin etc ???

Anyway...all the best with the project, and I'll be the first to raise a glass of the 'ole Janx Spirt when it's done......

looking at Rogers pix I think that a false bulkhead of some sort would be useful

especially if I am able to vector the engine

 

[Applescruffs]

Are you going to make a 'mock-up' out of 3/4 ply to test the strengh of the joints first?.........fix the engine in, let it dangle and give it a good wobble.....about 12 hours should do it.....and then check all of the joints, especially where the 'box' joins the hull.....

If all is well repeat the process on a daily basis for a fortnight .....

I'm thinking that an engine well that is part of the original specification of a boat will have been designed and built as an integal part of the whole structure providing all of the strength required...

I may be wrong, but I would definately make that test bed before I went too far up this road....

 

[prv]

I would definately make that test bed before I went too far up this road....

How do you make an accurate test piece that isn't exactly the same as the real job? There's no point testing a bare plywood box when the real thing will be encapsulated and strengthened by epoxy and glass and supported by being bonded to the hull

Pete

 

[dylanwinter]

How do you make an accurate test piece that isn't exactly the same as the real job? There's no point testing a bare plywood box when the real thing will be encapsulated and strengthened by epoxy and glass and supported by being bonded to the hull

Pete

I reckon I will get a day from the epoxy man

the idea is to make up asome think ply or hardboard templates fitted to the hull

then make the proper ones out of 25mm ply

it will have to be epoxied together in one hit

should be exciting

D

these are achilles wells

http://www.flickr.com/photos/8660415@N06/3391781735/in/pool-achilles24/

http://www.flickr.com/photos/8660415@N06/3393195272/in/pool-achilles24/

http://www.flickr.com/photos/achilles24/7514420222/in/pool-achilles24/

courtesy of Bert

 

[sarabande]

a bit of an anxious thought. How are you going to deal with exhaust fumes when the boat is stationary ? Coming up through the well into the cockpit might not be a good idea.

 

[dylanwinter]

a bit of an anxious thought. How are you going to deal with exhaust fumes when the boat is stationary ? Coming up through the well into the cockpit might not be a good idea.

I have not experienced that yet

the real problem comes when the boat is moving at the same speed as the wind - then it tends to hang around

the exhaust leaves the outboard down by the prop and tends to exit rearwards

incidentally, my little air cooled Honda warms the cockpit up

and here is a picture of an Achilles hole

http://www.flickr.com/photos/50731553@N06/5331491914/in/pool-achilles24/

 

[alan_d]

and here is a picture of an Achilles hole

Damn spellchecker. I am sure you meant to type "Achilles' heel".