Whale Tail / Hyrdrofoil plate design & questions - Laser 1750

[gmacrae]

CORRECTION: Boat is a BULLET 1750 (teach me for typing this up half pissed)

Happy new year to you lot!

I've made up a hydrofoil/whaletail plate for my little Bullet 1750 in an effort help keep the nose down at low speeds and take off a bit flatter. This boat's all weight in the ass and none up front, and although it has plenty of power to get up on the water, it would be nice to be able to cruise a little slower with the nose down (below about 25mph it's hopeless even with motor tucked right under)

So I made up this plate, which has made a big difference and drives great, but with the side effect of throwing up a wall of water behind the boat as you attempt to slow down again. The hull sits so low this could be a problem if you're not very careful and I don't want to sacrifice safety

Once above about 35mph this plate is well above the water and does nothing which is fine by me.

Does anyone have any ideas about changing the design to stop this tidal wave from trying to swamp the boat when slowing down? I was going to cut about 2" off the rear to see if it helped but I'm guessing here

Cheers!

Gene

[phillnjack]

the plate should be above water as soon as the boat is on plane, if the plate is touching water at 25mph then engine is far too low.

as for the wall of water when slowing down that is nothing to do with the plate, the plate will keep trying to lift the back up until about 5mph.

what your probably doing is having the motor trimmed up too much as you slow down, you need to tuck the motor fully under when slowing down.

at the front of your plate you could bend the ends upward like the Allison plates, this will also help the plate lift the back end...


just my take on it

[gmacrae]

the plate should be above water as soon as the boat is on plane, if the plate is touching water at 25mph then engine is far too low.

as for the wall of water when slowing down that is nothing to do with the plate, the plate will keep trying to lift the back up until about 5mph.

what your probably doing is having the motor trimmed up too much as you slow down, you need to tuck the motor fully under when slowing down.

at the front of your plate you could bend the ends upward like the Allison plates, this will also help the plate lift the back end...


just my take on it

The engine is running as high as it can, any higher will starve the water pickups. Like I said, once up to about 35mph the plate is well above the water.

The wall of water *is* to do with the plate, it didn't happen before the plate went on.

The engine has basically been tucked right in while trying this, it hasn't been trimmed up more than a couple degrees.

[Glastron1987]

There have been several models of hydrofoils that I have tried on my boat for some of the same reasons you mention, by I never tried making one. Good for you.

As far as that side effect, it is just a guess, but most of the ones I've tried do not have such distinct side flaps. My guess is those may be causing some of that side effect. Right now I am running the one from Bob's Machine, and I like that one. It has some flap-like things underneath it instead of on the sides.

[gmacrae]

There have been several models of hydrofoils that I have tried on my boat for some of the same reasons you mention, by I never tried making one. Good for you.

As far as that side effect, it is just a guess, but most of the ones I've tried do not have such distinct side flaps. My guess is those may be causing some of that side effect. Right now I am running the one from Bob's Machine, and I like that one. It has some flap-like things underneath it instead of on the sides.

Cheers, the main reason I folded the sides down was kinda to give it some additional structure/rigidity. I Guess I could take say half of the fins off though. I've seen similar designs (Permatrim, made here in NZ) that have the fins folded down at 90 degrees though and people seem to rate them highly. I think it's more of an issue with my hull than it might be with other boats - the back sits very low

[phillnjack]

where is the centre of gravity is for the boat and motor, you want to be about 1/3rd of the length from transom.
so if the boat is 18ft long then 6 feet from the transom the boat should rock easy on a piece of wood put between it and the trailer.

if your a foot out then the boat will play up and be wrong. just get the boat right to start with, then play with moving weight around.
.
NEXT
with boat sitting level to the ground (bottom of keel level with ground )check the transom to see how many degrees of tuck you are getting !!
might find you need wedges with a few degrees to get some decent tuck under due to transom not having much slant/angle...


is it a boat with foam under the floor ?? is that saturated at the back.
is the weight of the motor just far too much for the boat. what is the max weight of engine for the boat ?
where is the battery ?
where is the fuel tanks? is the tank far too bog ?
what other crap is stored backwards of the centre of gravity ( anchor , tool, fire ex, etc ).
where do you keep your ski ropes and ski stuff ?

is there a setback on the boat ? if so how much ?
every inch of setback is putting more weight past centre of gravity. a 300 pound motor set back 6 inches = 400 pound motor.
any wall of water coming up is from the gearcase of the engine, it cannot be the plate as that would be on a angle if the motor is tucked under making the transom
lift more with water going under it.

from the look of your top pic the boat is not capable of having that motor on the transom without having the problem of a wave coming up from the back when you slow down.


OR are you talking about The wave that follows the boat when you slow down ? two very different scenario's.
this is caused by the back end of your boat being too heavy, its exactly what people try to get for wake boarding, they put in wet sacks to add weight.
you need to lighten up the rear of the boat.. nothing else will ever work.
I have this problem and cant do anything about it due to weight of my boat v the engine i use. when I used a smaller motor the trouble was never there.


How long is the shaft of the motor ? is this a 25 inch mid on a 20 inch transom by any chance ?

I cannot believe you cant sit on plane at 17 to 20 mph and be able to safely see in front of you ..

[gmacrae]

where is the centre of gravity is for the boat and motor, you want to be about 1/3rd of the length from transom.
so if the boat is 18ft long then 6 feet from the transom the boat should rock easy on a piece of wood put between it and the trailer.

if your a foot out then the boat will play up and be wrong. just get the boat right to start with, then play with moving weight around.
.
NEXT
with boat sitting level to the ground (bottom of keel level with ground )check the transom to see how many degrees of tuck you are getting !!
might find you need wedges with a few degrees to get some decent tuck under due to transom not having much slant/angle...


is it a boat with foam under the floor ?? is that saturated at the back.
is the weight of the motor just far too much for the boat. what is the max weight of engine for the boat ?
where is the battery ?
where is the fuel tanks? is the tank far too bog ?
what other crap is stored backwards of the centre of gravity ( anchor , tool, fire ex, etc ).
where do you keep your ski ropes and ski stuff ?

is there a setback on the boat ? if so how much ?
every inch of setback is putting more weight past centre of gravity. a 300 pound motor set back 6 inches = 400 pound motor.
any wall of water coming up is from the gearcase of the engine, it cannot be the plate as that would be on a angle if the motor is tucked under making the transom
lift more with water going under it.

from the look of your top pic the boat is not capable of having that motor on the transom without having the problem of a wave coming up from the back when you slow down.


OR are you talking about The wave that follows the boat when you slow down ? two very different scenario's.
this is caused by the back end of your boat being too heavy, its exactly what people try to get for wake boarding, they put in wet sacks to add weight.
you need to lighten up the rear of the boat.. nothing else will ever work.
I have this problem and cant do anything about it due to weight of my boat v the engine i use. when I used a smaller motor the trouble was never there.


How long is the shaft of the motor ? is this a 25 inch mid on a 20 inch transom by any chance ?

I cannot believe you cant sit on plane at 17 to 20 mph and be able to safely see in front of you ..

Yeah, the slowest I can get the nose down is about 22mph (more like 25 without the plate), but the boat isn't travelling efficiently. At a guess, I'd say its centre of gravity (with 80L of gas) is about 2' forward of the transom (17.5' boat). It's all motor and fuel tank and bugger all else - pretty light boat. The pic in the water above shows it quite well (with ~250 kilos of people onboard/forward too)

The boat is set up (weighted) right, was set up by someone far smarter than me, someone who knows and races Bullets, and like I say, the "problem" has only existed since I put the plate on. You don't run any setback on these boats, from what I've read. I've seen bullet 1750's running 3L mercs and bigger yahama's etc, they're often set up very ass-heavy (more so than mine). 20" transom and mid.

Yes it has always had a wave form as you slow down, but it's never been as large as it is now with the plate. It was never a problem previously.

Hoping one of the Aussie Bullet guru's might chime in with their experience of running foil plates, they are a bit unique

[phillnjack]

foils are definitely not unique mate, millions of them around and tons of boats on here run them.
plenty of bullet boats have them and most of them are running jack plates with setbacks.
you need to shift weight forward by a good amount to make it ride nice at slower speeds.

looks like it has a pad on the bottom too ?
how far above bottom of the transom is the cavitation plate when engine is level ?

can you post a pic of the back of the boat showing the motor height ?
.

[gmacrae]

foils are definitely not unique mate, millions of them around and tons of boats on here run them.
plenty of bullet boats have them and most of them are running jack plates with setbacks.
you need to shift weight forward by a good amount to make it ride nice at slower speeds.

looks like it has a pad on the bottom too ?
how far above bottom of the transom is the cavitation plate when engine is level ?

can you post a pic of the back of the boat showing the motor height ?
.

Na I meant bullets are a bit unique, at least in these forums - there are a fair few here in NZ.

The cav plate sits about 3-4" above the pad when level, so the propshaft sits about 3" below the pad. I don't have a low water pickup but am working on it (have a sportmaster gearbox I need to have rebuilt when I have the $$) so will raise the motor a bit when I have that sorted.

There are 2 smallish batteries up the back (5-7kg each) either side of the central gas tank, but nothing else in the boat able to move. There ain't much to the old girl!

[powerabout]

you could go down the ballast tank route?

[phillnjack]

I thought your boat was a laser 1750, not a bullet 1750....

[gmacrae]

I thought your boat was a laser 1750, not a bullet 1750....

Oh jesus it's clearly been a rough week. My bad mate, I may have had a couple beers in me when I typed that, the motor's a laser 220, boat's a Bullet 1750. Not sure how the hell I got those wires crossed

[phillnjack]

No problem.. we all make cock ups all the time.
I see lots of them bullets sitting very low on the transom when stationary.

[StratosVT]

I think it would help a lot if you cut the plate so that you can slide it forward to the point that the back does not extend beyond the gearcase cavitation plate it is bolted to. The shape of your plate looks fine....it just needs to move forward a good bit. Good Luck!

Here is a pick of one I used to run:

[hsbob]

take a look at the hydrostream web site [the old history site]. in the race section are several photos of HS boats with fins on them. i have tested the doel fins and found they reduce the time to plane by a 1/3.. my feeling is if their good enough for racers the good enough for me.