extending hull

[schecter]

Can anyone tell me if it is possible to extend my propellertunnels and hull, to gain more lift at the stern, as my boat is "sitting" heavily?? I know it can be done in practice, but what effect on the boat handling will it make, rudders, props etc.

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[boatless]

Need a bit more info on the boat, but as a start...

Tunnels seemed to increase squat (design and tank testing patrol boats - first hand experience), the problem with extending is that unless you substantially change the rise angle from +ve to -ve there is no change in flow, on top of which you've just added more weight.

Best quick fix was transom wedges.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[schecter]

the boat is a 1989 Wellcraft san remo, 43 feet with twin cat 3208. My problem is that i need to have trimtabs fully down when boat is loaded, and topspeed is only 19 knots. The boat specification says 24 knots top speed which i cant achieve even with unladen boat. Im thinking of extenting the whole underwatersection of the transom to get more waterline and perhaps, a change in propeller

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[boatless]

Had a quick Google, but can't easily find any underwaterline images. Can you post a link to any?

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[schecter]

no, cant find one either, but it looks to me as any other boat 40-50 fett, with propeller tunnels. the dimensions is length 13.89meter, width 4.42 meter weight fully loaded 15.5 tons. It can be found on wellcrafts site.



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[boatless]

I can't really be too much help without some idea of how the stern/tunnel relationship looks. Wellcraft site just offers me a scanned newspaper shot?

First suggestion - is the bottom in good condition? Are you carrying a lot of weight in stores/water/fuel? Brochure speed will be dry/clean ship with empty tanks. Engines, serviced, clean filters etc?

Next. Props, need to check what you have as opposed to what might be a good idea.

Last, extending. You are not looking for more buoyancy here. It's dynamic lift you need. My first shot would be to fit wedges. Full width, chine to chine, 600mm long, 5 degrees down angle (measured from horizontal). Fair in around tunnels (you see my difficulty here?!).

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[schecter]

what exactly is a wedge?? You see im from denmark so technical english is a bit hard for me......
The reason i got this idea, is that wellcraft made a longer model called 46 motoryacht. it is extended with a cockpit compared to my boat. but as far as i can see, layout, engine location etc is the same. but this boat has better performance.

Im considering a steelplate from side to side of the transom, 2 feet deep, made to fit the hull and the tunnels. but unsure of the consequences......!!

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[boatless]

If this verbal description doesn't do it, I'll post a sketch. Here goes.

Ignore the tunnels for a moment.

Imagine two full hull transom flaps 600mm chord, bottom of vee to chine. Trim them so that they are an extension of the hull (a 1 Metre ruler lays flat on hull and flap in the fore and aft plane, now trim them to horizontal (in the fore and aft plane). Now trim them a further 5 degrees down. The flaps have now swept a volume of at least 5 degrees, probably 6 or 7.

Call that theoretical swept volume a wedge. Triangular prism. Now make up, in solid marine ply, a replica of that prism, and fasten it to the underside of the hull so that the back edge of the wedge is under the back edge of the hull. What it will do is act like a large planing flap, but without the expense.

Finally, the tunnels. They need to be treated in the same way, turning the flow of water downwards as it passes under the back of the boat.

On a series of patrol boat hulls we found that a wedge which measured 5% of the waterline length, at 5 degrees to horizontal, lifted the top speed by 12.5%. Figures were chosen because 'it looked right', and having achieved an improvement of that order, we were told we'd done enough and weren't allowed to run any more experiments at the expense of the Ministry of Defence!

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[schecter]

thank you, boatless, for your help so far. It seems like you know a great deal about the issue. is it possible for you to make a drawing?? im still in the mood of making the extension/wedge in stainless steel, formed to the hull/ tunnels, and now pointing downwards 5 degrees. the plate is going to be quite big, the boat width is 4.42 meters and then your suggested 60 mm deep. im also planing to support the plate to the underside of the the swimplatform, as i reckon there will be a lot of pressure on the plate carrying the boat and taking the pressure from the props

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[William_H]

You should make sure that all weight possible has been moved forward batteries fuel tanks etc. Is it crazy to suggest moving engines forward?

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[schecter]

the engines weighs 1200 kilos a piece, so thats not a great idea. it really is a problem that needs to be solved at the stern.

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[boatless]

I don't think that the swim platform will be strong enough.

The wedge goes under the hull, not behind it.

Lot simpler to make it in more or less solid ply and fix it under the hull, no bending moments. Also a great deal lighter. Weight is really not wanted here! The late and great Uffa Fox said that the only place to put weight is in a steamroller.

Happy to do a drawing, but I really would want to see some detail of this boat before doing so. Need to know what proportion of the hull is 'disappeared' by the tunnels, and how the tunnels interact generally with the bottom.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[schecter]

ok, now i think i got it: you dont extend the hull, but put a plate underneath the hull. that cant be done with the tunnels ? ive always thougt that the more waterlinelenght, the better performance. but youre right, its easier to put something under the hull.
the weight of the steel shouldnt be a problem though, and its an integral swimplatform, which maybe could be used as a locker after mounting the extension, as seen on recent princess constructions, where the hull and sterngear extends to the end end of the swimplatform. i cant give you a picture of the boat hull, but your drawing in any form would help to get the principle. i can then "adapt" to my boat.

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[boatless]

Sent you a pm. Look at the little red flag next to main index button.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[oldsaltoz]

G'day Schecter,

I have to agree that John [Boatless] is on the right track, I did a job for a client who wanted to fit a 200 HP outboard to replace his 150 HP, problem was the boat had very little freeboard at rest with the 150 and an extra 50 Kg was going to be major problem.

The boat had large trim tabs fitted but would always run with a list to port when the trim tabs were set to zero.

The client did a lot of skin diving and spear fishing.

I took some measurements and found the keel runner was 40 mm off centre, no doubt the cause of the list under power.

The end result was an extension of the hull at the transom, two simple boxes in essence, they conformed to the hull shape on the outside and below the water line, extending the transom by 450 mm with a small gap in the centre for the outboard leg.

The top of the extensions was kept low so that they could be used to get aboard from the water with ease.

The extended area provided 75 Kg of floatation and both had wedges built into the underside, the port side wedge was 33% longer and deeper than the starboard side.

Result: trims tabs no longer required so removed, boat now upright under full power, better performance, handling, and improved economy, easy to get back on board, rather than climbing up the outboard.

The wedges were shaped from closed cell foam and glassed onto the hull; some months after he was coming back from the great Barrier Reef in far from ideal conditions and the boat launched itself into the air doing better than 40 knots, when it came down it landed stern first and the bow slammed down soon after, they continued home at lesser pace, an inspection the next day showed no damage whatsoever. Just lucky I think.

So the bottom line is that wedges do work and work very well, can take a pounding and require no maintenance at all.

By the way John, I loved comment "Because they looked right" all that tank testing equipment and they went for the right look???

Very interesting thread.

Avagoodweekend........



<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[boatless]

I think you'll find that any decent Naval Architect goes for looks right first, maths second!

It's a long story, and probably still classified, but we used them new fangled computers to analyse the way patrol boat hull forms were heading. We then asked it to interpolate and predict 5 designs hence, then built a tank model. Didn't really need to wet it, the tunnels practically needed new superstructure to support them!

Filled them in, added the wedges, and because the bilges were a bit too round, bunged on some spray rails, off it went.... result.

Haven't revealed any of the classified bit - that was fun, politics n'all.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[boatless]

Oh, and ps. If you think sailing is fun, imagine being paid to play with 15ft plus toy boats...

<hr width=100% size=1>my opinion is complete rubbish, probably.