Hooked Hull

Kilted Kiwi · 25 November 2003, 07:41

Hi
Hope some one can help. I have just upgraded a pair of 90 mercs to a single 175 EFI on the back of a Naiad 5.8m. I had been expecting great performance imporvements, 85kg lighter on the back end and with only one leg dragging.
Top speed is much the same, as is economy, the boat now seems to ride too low at the bow as if I am trimmed too far in. I now have to run fully trimmed out all the time, and load all my gear in the stern.
A friend suggested that there may be a hook in the hull but I cannot see it.
Is there any way to check this and if it is hooked is there any way of fixing it.
Any suggestions on how to get the nose out of the water would be greatly appreciated.
Cheers

Cookee · 25 November 2003, 08:18

I am absolutely certain someone will come up with other suggestions, but:

You will have altered the centre of gravity by removing 85 kg from the transom, therefore moving anything you can towards the back of the boat is essential to keep the same C of G.

Have you changed the type of propellor as well? Some props have bow or stern lifting characteristics, so that could be influencing things as well.

If the hull had a hook in it I don't think changing engines would have made it suddenly affect your boat in this way (IMHO)

If the boat was built for the twin engine set up, maybe the builder / rigger put the console further forward (does it have a console?) or installed items such as the battery etc further forward?

My opinion would be to move everything to the transom, try a prop with more bow lift, and if that doesn't work fit some doel fins or similar to the cavitation plate!

By the way what is a Naiad 5.8m anyway?

Good luck,

nik · 25 November 2003, 09:21

Its one of these.

Try http://www.naiad.co.nz/

Dont bother contacting them though, I have tried twice, no reply yet.
Nick.
(hopefully the pic will come out)

[IMG][IMG]C:\My Documents\My Pictures\058SportLine.jpg[/IMG][/IMG]

nik · 25 November 2003, 09:37

Or how about one of these

http://www.naiad.co.nz/Gallery/NaiadExplorer.html

[IMG]C:\My Documents\My Pictures\16-7_1.jpg[/IMG]

geoff · 25 November 2003, 18:50

Dead easy to see if you have a hook in the hull. When the boat is out of the water get you eye level with the bottom of the transom and look along the keel. Even the slightest hook will show up although I doubt this is the problem as it would have to be pretty bad to cause what you describe.

Kilted Kiwi · 25 November 2003, 21:04

naiad is the same as nik's first image not the monster.
Can't see a hook from the transum. When the engine was fitted I tried a range of merc props, and ended up with a 17" pitch Vengeance, couldn't use anything bigrer or go to a 4 blader as revs dropped off to 5300 ish!
There's nothing to move aft, there is a basic cuddy cabin up fwd where controls are, which is otherwise empty, internal 200ltr fuel tank running form a well at the stern to 3/4 of the way fwd, battery is under drivers seat.
Thought about dolphin / doel fins on the cav plate but thought they were for getting the boat on the plane quicker, thus biringing the bow down!
Should have said the hull is 5mm Aluminium and weighs about 450 kg + engine.
Cheers again.

Cookee · 26 November 2003, 07:51

Kiwi,

I know it's a pain in the ass but see if the battery will move to the transom in a box?

Have you got a huge anchor and 3 miles of chain in the bow?

What's in the cuddy - if you need it all just remove everything as a trial just to see what happens - you might not need to move too much too move the C of G back.

The fins also make your trim more effective as well as getting on the plane quicker, they only stop the bow going so high in the air when the boat is getting over the hump, just a small change in trim makes a huge differance.

Hope the above helps -

niels · 26 November 2003, 11:33

try a tempest plus.
it has a good bow lift and good top speed

Robert1 · 18 December 2003, 20:57

Geoff,

You describe how to find a hook in the hull... look along the bottom of the keel from the transom. But what are you actually looking for? Will the hook show up as a tiny deviation or a obvious 'bend' in the hull?

Robert

jwalker · 18 December 2003, 21:20

It bends downward towards the stern. Usually, close to the stern.

Robert1 · 18 December 2003, 21:23

How easy do hulls get hooked? Can it be done easily or does the boat need to be dealt some serious abuse?

Robert

jwalker · 18 December 2003, 21:32

Sittting it on a roller trailer before the resin is fully cured will do it. Although polyester resins are supposed to be thermosetting, some are also a bit thermoplastic. Strapping a boat down to the trailer really hard and leaving it there through hot weather may also do it.

Some boats are built like this to keep the bow down.

Robert1 · 18 December 2003, 21:34

Not just by sitting on it then!

Thanks JW

jwalker · 18 December 2003, 22:01

Quote:

Originally posted by Robert1
Not just by sitting on it then!

If you mean boat on trailer, yeh, could do, if the moulding/materials are crap.

I once built a slalom canoe from materials sourced from the local laminators. When it was strapped to the roof rack, and the sun was shining, the ropes left grooves in the deck! A hot air gun removed them! Very thermoplastic - with a memory, like acrylic.

Didn't buy from him again.

Pete7 · 18 December 2003, 22:10

Quote:

the ropes left grooves in the deck! A hot air gun removed them! Very thermoplastic -

Brilliant, a canoe with a stepped hull, should have really shifted

Surely the problem with the boat is 150 kg of GRP cuddy on the front of a 5.8m hull, ie a short one. Think Cookes idea of moving things aft makes sence.

Pete

geoff · 18 December 2003, 22:40

jwalker is absolutely right. The point is that when you look along the keel of the boat you can pick up even the slightest deviation but I believe that it has to be pretty bad before it affects the handling.