Hot Boat

On a single engine V with a 7" pad on the bottom. Where should the prop shaft center line be? Or more precisely, the vertical centerline of the drive in relation to the vertical centerline of the transom

I would think center to center , but I was told the some boat might have the drive off set to one side to counter act tork.

I am asking because I am looking for a reason for the high slip #'s I have and was wondering that if the drive was off center would that be a cause for added slip.

The boat is an 88 28' scarab. I know the hull was not intended to go as fast as I have pushed it but 23% slip at 85mph seems high.

I know that the bottom of the boat needs some work to make it smoother. It has a lot of small high and low spots that need to be lightly filled and sanded out in order to be smooth. I know that alone is holding me back some but I am curious if the drive might be an issue as well.

monstaaa

if your asking if it should be off center of the boat? the answer is no.
if its a height question, than i would think it has to due with power, speed, and center of gravity.

to high of an x dimension will produce a lot of slip. i put a -2 inch lower on my f1 boat. i was able to spin a bigger prop, but with no speed increase.

Hang Time 27

What prop are you using? More blades may help with the slip, but will also put more stress on your drive.

Maybe you've already looked at this:
If you have any hook in the hull, it will cause you to run more trim angle which will cause higher slip percentage. Those hulls have a nice hook at the back of the outside strakes, but at those speeds you're probably running on the pad only, so I'd check that - any downward hook at the transom (or upward curve in the last couple feet before the transom) will cost you trim angle, which will cost you speed and increased slip. Run your slip #'s at lower speeds, maybe 70's, and see if they are any better - if so, and you are trimming up more to go to 85, then it may be the bottom.

Hot Boat

I have some nice Heli photos of the hull at high speeds and it's running on the pad like it should.

The slip #'s are much better at 75mph. rpms are 4600

When I increase rpm from 4500 to 5500 I must trim up more to get the best possible speed.

Even at 4600 rpm (75 mph) in order to get the best possible speed at that rpm I through a 4'-6' high rooster tail. And at top speed I'm told its 8’ high and 40' long

We tried all kinds of props. 4 blades and 5 blades. The best handling and speed was with a 32 B1.

My drive center line is about 4" below the pad.

monstaaa

do you know where the c.g is in the boat? if you have a straight bottom, no hook, or rocker, and your spining a 32 i might think you have a weight issue. it sounds as if you cant carry the bow. is it a standard lower.?

Pure Energy

Too much weight......uh oh...better start that diet!!!

I'm tell'in ya, drop it down an 1.5"

Hot Boat

As stated before I know that the bottom of the boat needs some work and that might be my problem.

But my original question is.... Is the vertical center line of the drive always in center with the horizontal center of the transom???

And if they are off will it cause slip?????


Monstaaa, below are some photos of the boat running a 32 B1 at WOT. It looks to me that it carries the bow but I'm no expert. The boat has a 7 " wide pad on the bottom that starts about the steering collum back. The drive is an Imco 2" lower set at 5 1/2 " below the bottom of the hull, But I have a stern jack that alters my trim angle. It will give me a higher X dimension with lower angles of trim than a normal set up.

I don’t know the CG of the boat. I can tell you that the cabin is for the most part empty. It’s carpeted but it’s been gutted. Most of the weight is to the rear.

In the next few weeks I’m going to do some work to the bottom of the hull, Every time I did some work to the hull the boat rides better.


Pure Energy. already did LOL

Hot Boat

If you look closely at the photos, you can see where I started to sand the bottom of the hull to make it straighter. I used a 2" wide by 4' long oak stair rail that a glued sand paper to. I originally wanted to sand some of the hook out of the boat but what I found was lots of high and low spots all over the bottom. You can see the rougher sanded areas in the photos.

Do you think this might be my problem????


You can also see the modified stern jack I have on the drive

monstaaa

pure energy may be right. i have set up a couple of slightly larger vee's with adequate power rand the were enhanced by lowering the drive.
have you ever tried a standard xr with dual water pick ups.?

Hot Boat

Yes. Not an XR but a stock bravo with the 600 hp motor I had before.

I'm not sure what if at all any speed gains I got with the 2" shorty but the over all ride of the boat was 100% better. My wife insisted that I keep the 2" shorter drive on the boat.

If running the 2" shorter drive is my problem I would rather keep it on the hull for the gains in rough water handling. It’s a mater of keeping the wife happy. But I would like to try a drive spacer and see if it helps just for piece of mind.

Here is a copy of the post that got me thinking if my drive is off center. I haven’t checked yet but I'll bet $$$ it’s off.



Pantera 28-650,

I have a friend of mine, of which he has a 28 foot boat (UK phantom now called Chaudron) powered with 600 hp. This boat wouldn't run more then 67/68 mph with 22% slip. We tried diffrent props like 3 and 4 bladed, but still no luck. Then we turned on the gear ratio and went from 1.36 to 1.50, changed and tried diffrent props still the same. So our last goal was to try a spacer. Got the spacer 1 1/2" and still we had high slip numbers with no extra mph.
One day, we were going to the other Island to spend the weekend and from my boat I noticed that his boat was leaning to starboard. When he took up the boat I told him to check if the drive was fixed in the center of the boat and he found it 3/4" to 1" off the center line to the starboard side.
We arranged the drive to center ( easier said then done) and now the boat runs 76 mph with 12% slip.
I don't know if you have the same sort of problem but there are alot of similarities.

Just my 02cent.
Good Luck in finding the problem.