Need help with drive heights

kubcat · 02-19-2006, 12:24 AM

I tried posting in the general Q & A but they suggested I try here.

I have a Force 3500, which I have been told is an almost identical hull to a late 80's 36? or 38? Cigarette. She was originally built as a race boat in Australia but just before completion she became a cruiser. She was fitted with 4 foot long pods and twin outboards. Weighs about 7000 pounds with no fuel. I don't want outboards anymore - personal choice and I have purchased twin 496HO's and Bravo X drives. I am ready to cut some holes in the transom but have no idea where.

The boat was laid up for twin big blocks and arneson drives according to the retired boat builder but the stringers in the boat appear set up for a single installation. Guess I have some glassing to do. I have "engine" hatches. but the stringers appear to be set for a single installation.

I guess the questions are:

1) Does mounting the engines 35 or 36 inches apart make a significant difference;

2) What X dimension did Cigarette use and/or what should I aim for:

3) What size stringers should I build?

Any photos of empty engine bays would be fantastic.

CignificantOther · 02-19-2006, 03:52 PM

Do you have any pics of the boat/transom? Notched transom or standard "V"? Deadrise?

In general, the closer the drives are mounted, the more efficient the propulsion and the lower the center of gravity. That is, the hull can lift as speed increases creating less resistance while still maintaining adequate/full thrust from the props.

Drive height will also depend on the c.o.g. of the hull. If the hull is bow heavy, keep the props deep enough for leverage to lift and carry the bow. If you don't have any issues trimming the boat, a higher drive height should increase speed by reducing drag.

kubcat · 02-20-2006, 04:22 AM

Quote:

Originally Posted by CignificantOther

Do you have any pics of the boat/transom? Notched transom or standard "V"? Deadrise?

In general, the closer the drives are mounted, the more efficient the propulsion and the lower the center of gravity. That is, the hull can lift as speed increases creating less resistance while still maintaining adequate/full thrust from the props.

Drive height will also depend on the c.o.g. of the hull. If the hull is bow heavy, keep the props deep enough for leverage to lift and carry the bow. If you don't have any issues trimming the boat, a higher drive height should increase speed by reducing drag.

The boat with the outboards had no issues with trim except it took a while to plane, not as bad as some of the race boats that I have been in, but not great. The pods are mounted almost 4 feet from the transom, so I am not sure if I can use them as guide for height. We would get just under 60mph with twin 275hp V8 Evinrudes.

The transom is standard, not notched. Not sure about the deadrise, I will look for my high school protractor and measure it.

Meanwhile there are some photos on the way.

kubcat · 02-20-2006, 04:28 AM

Some more photos

kubcat · 02-22-2006, 07:19 AM

Can anyone with a 36-38 Cig, non step, standard transom measure the height of the prop shafts when trimmed level to the bottom surface of the boat immediately ahead of the drive? I am thinking it should be somewhere between 4 or 5 inches. Any help would be appreciated as I am about to start cutting my transom.

CignificantOther · 02-23-2006, 11:08 PM

My 38 with a notched transom and # 6 Drives is 1/8 " below the hull.

My 35 Mistress with no notch and # 3 SSM was approximately 3 1/2 -4" below the hull.

Hope that helps a little..........

kubcat · 02-24-2006, 12:02 AM

Quote:

Originally Posted by CignificantOther

My 38 with a notched transom and # 6 Drives is 1/8 " below the hull.

My 35 Mistress with no notch and # 3 SSM was approximately 3 1/2 -4" below the hull.

Hope that helps a little..........

It helps a lot. Guess the 4-5 inches I was thinking is way off. Thanks.

GLH · 02-24-2006, 07:34 AM

CigOther's number would be your best input.

Also I would email or PM Fred at Trick Marine he had a Cigarette that was in those dimension, he could get you in touch with the person that bought it and they could take some dimendions for you. Actually anyone here with a Mistresse Cig could give you input.

Hope this helps.

Good Luck with the project.

kubcat · 07-24-2006, 08:40 AM

We have had some delays, but the boat is at the glass shop and we are about to start drilling.

Looks like we will be aiming at 4 inches. If anyone with a 35 or 36 wants to run a tape measure on their boat or if anyone has anything to add, now would be a great time...

Thanks

Kubcat