sportfire / diplomat hull questions...

[wca_tim]

for an outboard about what speed do these hulls really air out? I'm assuming that the 4.3 version i/o's don't really

I'm playing with set-up on my diplomat and even though we're moving along really well already, it is clear that it's never really up on the pad.

I'm running a smallblock that should be within 50 pounds of the 4.3 that was originally in it.

I'm doing some cosmetic work here and there and was going to blueprint the pad and paint the bottom this week, but I'm really tempted to widen the pad just a little...

Any thoughts or experience? or even pictures of sportfires or diplomats running at higher speeds (ie 80+)?

Thanks!

 

[Mark]

Oddly the outboard and I/O are actually not the same hull. The I/O is over a foot longer and much heavier with the motor.
The outboard Diplomat/Sportfire is on pad very easily...maybe 40 MPH. By 55 it is chinewalking. You could get it to 80 with the right power but honestly it would be a wild ride.
I would think all the I/O hull needs is just more power and it would ride nicely on pad.

 

[wca_tim]

Thanks for the info Mark.

I wondered about the hulls being different, it just feels like the i/o carries it's weight awful far back and when I trim it out to the point where I hit the highest speeds it feels like it's "loose" on the very back of the hull, but doesn't feel at all like being on tha pad based on my experience with a hydrostream some years ago. It just seems like the hull should be a bit faster than it is... but then again, I'm the eternal optimist. I can run at mid 60's or better with any of the props I've tried and hit as high as 75 with one particular prop. so far other than feeling a little squirrely and having issues with mid range porpoising with bow lifting props before I put tabs on it (that was the same with the v-6), it's been very well mannered. It sems like I should have the power to go faster and like a lot of the hull is in the water at speed... I know it's no light little hydrostream hull with a screaming outboard, but it just seems like it is not getting the full benefit of a padded hull design to the point where it will get up on the pad and "fly" I would also think I should have the horsepower to get it going faster... I know the motor and drive are a lot heavier on the i/o and was thinking that the pad width is just not sufficient to fully carry the weight out of the water. Was thinking that by using some appproximate weights and maybe having some pics or movies of the same hull design all the way up on the pad to get a good idea of how the hull carries different weights I could find a basis to estimate how much additional pad surface area might be enough to carry the added weight at a similar angle. The pad width / geometry is one of a few things I'm questioning or thinking about playing with. It may be that just removing the rocker that is in the rear section of my hull now will make a bit of a difference...

 

[Mark]

You are right, an I/O will never 'fly' the way an outboard like a HST will. Even the ocean racers doing 130 MPH only fly when they hit a wave. My neighbour has a Eliminator 25 Daytona with 525hp. 94 MPH and it still looks like it is glued to the water.
Widening the pad is a roll of the dice. There is a Pulsare 2100 on the board with a 10" pad that is 5 MPH faster than similarily powered rigs (TCAT). But most pad widening projects that I have followed have been a failure.
If you are seeing 60+ with a 4.3L you are doing very well. I'm not sure I believe your 75 MPH run. LOL

 

[wca_tim]

uh... sorry the post wasn't clear...

I could hit 60 with the right prop, a little chop and a light load with the 4.3 in it... (carefully tuned non-balance shaft 4.3 with vortec heads, roller cam, edelbrock 4-bbl and intake, etc...)

The 75 is with a 383 stroker small block. I had all the machine work and long block tweaking fitting, etc... done by a local shop with extensive marine / offshore and circle track experience... We didn't dyno the engine, but based on the many engines of this general type he had previously built and dyno's, we added at least 200 horsepower as it sits in the boat (accounting for marine manifolds, spark arrestor, mufflers, etc...)..

I'm running the original 1.81 gear ratio to make use of props with roughly 2:1 pitch to diameter ratio and realize that if I hammer the alpha drive or run flat out for long periods of time the alpha drive is living on borrowed time... I tried a 1.47 gear upper I have (had.. lost a u-joint), but as predicted, performance wasn't as good on the top end. I've put a temp thermocouple in the upper drive to see what temp is doing, but haven't installed the gauge yet.

I'm going to try a mirage plus 27" for general skiing and so forth (have one being shipped now), but the best top end was with a hydromotive quad 4 29" running 5200 rpms and 75-76 mph (gps) in light chop with two people, full tank of gas, skis and tool box on board. I have no problem pulling wakeboarders up with the hydromotive and it hand;es like a dream, but it's a solid hub prop and running on the river where we do, there's always stuff to hit things...

I've done some reading on boat and hull design and looked at the bottoms of a number of boats and so have some pretty specific (and not too radical) things I would connsider trying on the pad, but won't bore you with the details of my silliness...

sorry to ramble... it's one of htose days when work just doesn't seem appealing...

Thanks for your perspective / ideas