Water rise over run @ 70MPH??

[kimswang]

How much will the water rise from the transom to the engines at 70MPH?? My engines have a set back of 35" and I have numbers ranging from 1" up for every 1-1/2', to 1" up for every 5" back. Any help here will be highly appreciated as time for testing my triple V8 setup on my Scarab 34SS is about to happen.

[markt111]

Hey Kim, I always thought it was 1" for every 5-6" setback, but I'm not the expert, did you raise those outside motors?

can't wait to see your results, sh** can't wait to start playing with mine again!!

[kimswang]

Hey Kim, I always thought it was 1" for every 5-6" setback, but I'm not the expert, did you raise those outside motors?

can't wait to see your results, sh** can't wait to start playing with mine again!!


No, haven't done anything and had decided to give them a test run first. However, Keith just posted some awesome video of his rig running, suggesting a 5.25" rise over a 30" run (setback). With this data, I have to lift all my engines 5" and put a 5" extension on the mid engine. This implies a whole lot of modifications to my already modified bracket. I have to finish up all my wiring today and then move on to getting the engines ready. Should be testing within short period of time here..

[hsbob]

rule of thumb was 1/8 per inch of set back. but some of the hydrosream for example have a hook built in so use common sence.

[STV_Keith]

Kim, I corrected my previous statement after talking with Kevin yesterday...1" to every 8" back.

Also remember that the boat will not run flat, as it sits in the yard on the trailer. It will run bow up (with all that weight on the transom), and with the pivot point being forward of the transom, that means the engines will actually be even deeper.

[Hottrucks]

rule of thumb was 1/8 per inch of set back. but some of the hydrosream for example have a hook built in so use common sence.


YEa what he said is what I use for rule of thumb

[Cobra1racer]

Best thing to do is have some one drive the boat while you are at the back looking down to see where the motors really are. How else are you going to know where you are at without looking, Guess?

[STV_Keith]

C1R, his boat hasn't hit the water yet...he's trying to do a best-guess before water testing.

[Cobra1racer]

Well then just take it out and see where it's at. I know for my little ride I did 1" up for every 3" back. At 65 the cave plate was an inch out of the water. And this was with a 12" set back. I want to see the vid Kieth.

But keep in mind Kim If he has a setup like that on a tunnel he is riding on top of the water, were you, sorry, won't be. Two different animals with a different X demention for the drive higth. Just set it up were you think it will run and see from there. I'll bet money on it that you will be higher than the five that you think you need. How high are you running right now from the 25" mark on the transom to the new location on the setback?

[kimswang]

Well then just take it out and see where it's at. I know for my little ride I did 1" up for every 3" back. At 65 the cave plate was an inch out of the water. And this was with a 12" set back. I want to see the vid Kieth.

But keep in mind Kim If he has a setup like that on a tunnel he is riding on top of the water, were you, sorry, won't be. Two different animals with a different X demention for the drive higth. Just set it up were you think it will run and see from there. I'll bet money on it that you will be higher than the five that you think you need. How high are you running right now from the 25" mark on the transom to the new location on the setback?

Right now they are even height, mid engine being propshaft 2" above the bottom, and outer engines being way deep as it has a 24 degree bottom, sits 28" apart and should equal aprox 11" difference. Thought I would be ready for a water test tomorrow, but got too much wiring left for that to happen. Now I am also going back to work, so I have some time to do a little "fine calculations / ideas" to solve the problem. I am just trying to figure out a good starting point and I am willing to give up the 2MPH and not surface the props. I guess there will be several test runs with adjustments made between each, just trying to keep the down time to a minimum.

[Cobra1racer]

Good Luck.

[STV_Keith]

I am willing to give up the 2MPH and not surface the props.

Gonna be a heckuva lot more than 2mph. I'm still not surfacing my props and a 2" rise in drive height just equated to just shy of 5mph.

[markt111]

Hey Keith how many RPMs did you gain with the 2"/5mph raise??

finally got some picks of my center console today will put them up later!!

I guess us Scarab guys keep picking each others brains

[STV_Keith]

4.7mph, ~350rpm or so. I just raised the motors another 2" today. Will water test tomorrow.

[kimswang]

4.7mph, ~350rpm or so. I just raised the motors another 2" today. Will water test tomorrow.

You are doing great Keith. You are a few steps ahead of me. I have some battery cables left to hook up and I will go to the lake as well. Finished up the wiring in the console wich turned out to be quite a job with all those gauges, plus my own trim / tilt system as well as my warning / alert systems and charging systems that I made. As you can see, a lot of wires. Will spiral wrap it all after the first sucessful test. If you read this before you go to the lake - take as many pictures and measurments as you can. Maybe the 7" lift for mine is a good idea after all. Can't wait for the results, get mine dialed in, and have a killler good time on the water.

[STV_Keith]

Looks great Kim! Sorry, didn't get any pics today. Boat didn't do any better than it did in Havasu, but it was a little warmer, and it's 800' higher elevation here, so...

I'm pulling the heads tomorrow and will be adding compression next. Then, at this engine height, I will re-test in Havasu the weekend of the 28th.

[kimswang]

I will re-test in Havasu the weekend of the 28th.

If you only knew how hard I am trying to get off that week end. I have posted my trip and I am only hoping that someone will pick it up. Lets keep our fingers crossed..

[supergenius74]

Boy that scarab must have a very strong transom to hold 3 v8's at 35".

[kimswang]

Boy that scarab must have a very strong transom to hold 3 v8's at 35".

Every scarab is strong, what do you mean with 35"??

[supergenius74]

35" of setback, that's huge, I thought that 13" on my boat was a lot, puts a ton of stress on the stransom. When I calcutlate out my set up I have a 400 lb motor at 1 ft I get 46.7 lbs per sq in of force on the transom, when I calculate out this set up we have 3 motors @ 500 lbs at nearly 3 ft = 525 lb per sq inch, thats huge. Hence my comment about the scarab transom being very strong, I have no idea how thick it is but it must be very thick, thicker than the boats I am used to that are 3" thick.

[kimswang]

when I calculate out this set up we have 3 motors @ 500 lbs at nearly 3 ft = 525 lb per sq inch, thats huge. Hence my comment about the scarab transom being very strong, I have no idea how thick it is but it must be very thick, thicker than the boats I am used to that are 3" thick.

I don't really know how your formula works. Doesn't pressure pr. sq/inch have to do with area?