Hull Modifications, Advice wanted.

I’m looking for some advice from anyone with experience in hull design/ boat builders etc. As you can probably tell by my terminology I’m not a boat builder, but I can weld. So I was planning to do this myself.

I have a 5m Aluminum run about and I want to put on a boarding platform on the transom.
And I was advised by a friend (who has done this) that if I make a sealed pod for each side (instead of just a flat platform) That it would not only make a good boarding platform it will give extra buoyancy and increase the boats waterline length. Helping it plane better.
I hope that’s a clear enough description.
The outboard is mounted on a bracket (for want of a better word) not the transom.
My boat has a relatively low freeboard and sits a bit low at the back when people are fishing back there.

I have some doubts though, because if it was such a good thing why don’t they come with them fitted?
And the guy who sold me on the idea spent good money on getting his done and would probably downplay any negative effects it had.


So has anyone done this? Or know why the boat builders don’t put them on as a factory fit?

Any advice would be appreciated.
Thanks

 

Here at Explorer Marine in the UK - see www.explorermarine.co.uk - we make most of our RIBs with " extended planing area" we sell this to customers as boarding steps or swim platform, but the main reason is to extend the hull behind the outboard which gives us a much more rapid acceleration - very useful on ski boats. The extra area and change in L C of G also means that the boat accelerates without as much bow lift which improves visibility. Thirdly, because of the extra buoyancy and the extra planing surface length, we can fit larger outboards. For example we will fit the ETEC 175 hp on a 5.5 metre RIB, and the 225 or 250 on a 6 metre RIB, which is then good for 65 mph

I do not know how good your welding is, but we find it useful to install drain plugs to drain any water from the platforms - GRP in our case - and we foam fill them anyway.
I can send you some pictures if you have an email address.

Good boating - and a happy New Year

Hugh Mattos
Chartered Engineer

 

Thanks for the reply,Some pics would defernatly help. lukeandbrigid@yahoo.com

My main concern was water flow past the outboard and possible cavitation during a turn?

And was wondering why they are not that common?
My guess is maby if a manufacturer wants a certan waterline length they build up from there to maximise the volume inside the boat?

 

If well designed, the additions you are proposing are very effective. They work like large trim tabs, but are not adjustable for speed or loading changes. If the planing surface of the extension is a simple extension of the boat planing surface, then at high speed, the boat will most likely plane at too low a trim angle.

The reason for them not being standard on most boats is the cost of getting the design correct and the fabrication cost is more than simply lengthening the boat.

These extensions allow low draft, low planing speeds and significant stability improvements in addition to the access benefits you were interested in.

 

"If the planing surface of the extension is a simple extension of the boat planing surface, then at high speed, the boat will most likely plane at too low a trim angle."

Im not 100% sure what you mean by too low a trim angle?
Do you mean that the boat will ride bow down? If so surley I can compensate by triming the outboard?

 

Gulf Cruzer, I have sent you a series of photos by email, and as you can see the boat does not run trimmed too far down as we have plenty of lift all along the boat.
When we discuss this feature with customers we describe the design as making a 5.5 metre boat think it is a 5.75 metre, hence the better handling. As the engine moves forward relative to the back end of the planing surface by 250mm the boat handles in a much calmer and more pleasant manner.

Cavitation can be an issue on high speed turns - which we do during demos! Our particular favourite is full lock at idle then full throttle to show how stable the boat is.
On our hulls we fit 20 inch engines which is 508 mm, but we fit them to a 535mm transom as we find that is the best compromise between top speed, economy and cavitation. For high speed cornering we trim the engine right in, but for general running at speed we teach our customers to trim the engine out by a few degrees....

Good Boating.

 

Thanks for the pics Hamattos and nice boat.

That is pretty much what I had in mind for my boat.
How bad is the cavitation? Can the boat still turn at high speed? or is it only when you put alot of wheel on?

 

Yes, trimming the outboard will help to raise the bow, but efficiency will be decreased if you are forced to excessively trim the motor to compensate for having too much lift behind the transom at high speed.