My new too me boat questions

[bluefin59]

Now iv had the boat a few trips in now and a bit of adjustment to transducers and new steering ram also a sender unit for the fuel tank I have a few questions , the boat and motor seem well suited . I’m seeing I don’t need much trim and when I do give it a bit it dosent seem to need much at all but sits quit flat to the water and I’m happy with that as well it’s obviously a different sort of hull to the bluefin , the issue I have is cavitation on turns that are tight and it does it fairly easily . It’s 75hp with a 4 blade spitfire prop which obviously helps with lift because this thing is straight on the plane travelling beautifully, do you think a wing thing needs to be used to help with cavitation or am I dreaming

[Noelm]

Lots of tinnys with that little keel extrusion will ventilate on turns, some props are better than others. Dropping the motor one hole might be worth an experiment, but, it’s a coverup, if it’s the prop that’s the fault. Be nice if you could borrow a different prop for testing purposes

[Dignity]

On a hard turn you should trim the motor all the way in, also a 4 blade prop by it's nature will cavitate if you don't.

[tunaticer]

What is your hull?
Does the keel finish at the stern or how far forward does it finish?
If it gets up well and handles nicely do not throw a foil on.....investigate depth and trim.

[tunaticer]

duplicate post

[bluefin59]

Here are some pics for perspective as to how it’s all sitting , I don’t want to put a fin thing on if I can help it and may have to put up with a little cavitation on hard turns . It dosent have much trim that makes any difference,I trim it out a touch and anything more it dose the bouncing thing at the bow a little but it certainly dosent lift the boat at all it just sits flat and comfortable using the hulls shape from what I can gather . Thanks Matt
It’s a 20 year old JBS marine plate hull , the motor pulls 5300 rpm flat out

[bluefin59]

On a hard turn you should trim the motor all the way in, also a 4 blade prop by it's nature will cavitate if you don't.

I have very little trim when on the plane but I will try it with it trimmed right in , thanks

[Noelm]

That’s a pretty common motor, be nice if you could borrow a “standard” prop from someone just for a test run. The Merc 75 is a heavy motor, maybe the 4 blade was fitted because of the weight?

[Dignity]

Matt, the height could be reduced from looking at that first picture. It is hard to tell from pics and you should have the motor trimmed out in running postion with some sort of straight edge under the hull to get a better idea.
The best way is while under way have someone go down astern and see if you can see the anticav plate, it should be just on the surface, too visible it's too high, not visible it's too low. If they can get photos it would help.
If it's just right then you need to look at changing your prop. On hard turns your prop will be getting more air than usual and a 4 blade will cavitate earlier than a 3 blade. A 4 blade is used for hole shot mostly to pop you out of the water quicker but with loss of top end speed. This is very useful for crossing bars, skiers etc. As Noelm says do try a 3 blade if you can get hold of one, solas props used to loan props for testing, not sure if they still do and possibly there was a change of ownership.

[bluefin59]

Matt, the height could be reduced from looking at that first picture. It is hard to tell from pics and you should have the motor trimmed out in running postion with some sort of straight edge under the hull to get a better idea.
The best way is while under way have someone go down astern and see if you can see the anticav plate, it should be just on the surface, too visible it's too high, not visible it's too low. If they can get photos it would help.
If it's just right then you need to look at changing your prop. On hard turns your prop will be getting more air than usual and a 4 blade will cavitate earlier than a 3 blade. A 4 blade is used for hole shot mostly to pop you out of the water quicker but with loss of top end speed. This is very useful for crossing bars, skiers etc. As Noelm says do try a 3 blade if you can get hold of one, solas props used to loan props for testing, not sure if they still do and possibly there was a change of ownership.

Ok that makes sense it jumps on the plane amazingly not like any boat iv had before, I will trim it out a touch this afternoon and put the level on it and take a photo

[Flex]

Engine looks like it could come down a bit.

The best test for correct engine height from my experience is how much blow out you get in a following sea when trimmed out enough for decent handling?.

you dont want the engine ventilating in a following sea and not be able to trim it out enough to stop it keel/bow steering.

[Out-Station]

Bluefin, other thing that gives you a feel for scope to move lower thats usefull to add onto the ideas above is when your up cruising on the plane flat water from full "in trim" how much out trim can you get before you get into some ventilation issues. For me i work on about 2/3 of the trim range without allot of ventillation as being good and usually agrees with cav plate positions etc.
Scott

[bluefin59]

Bluefin, other thing that gives you a feel for scope to move lower thats usefull to add onto the ideas above is when your up cruising on the plane flat water from full "in trim" how much out trim can you get before you get into some ventilation issues. For me i work on about 2/3 of the trim range without allot of ventillation as being good and usually agrees with cav plate positions etc.
Scott

I will see when I go out next what sort of trim it starts ventilating, this thing is way different to any boat iv had before . It’s solid on the water and barely lifts the bow when I trim it , I don’t get a drop of water in the boat at all when travelling it’s really different

[ericcs]

my 5m quintrex/90 merc(no foil), was behaving very similar to yours. dropping it 1 hole made all the difference, much more trim, and way less cavitation!

[chris69]

If you drop it down it will be interesting to see if you loose any revs that 4blade props have some pitch i see your prop is 14x29!!!.