changing from outdrive to prop shaft

[rich99uk]

Can anyone one advice me on the following

I have a ford transit engine 2.5 65 HP which was on a volvo penta 270 outdrive the ratio on the leg is 2:1, on a 23 foot fishing boat semi displacment hull and very heavy boat about 2.5 ton Max I have now decided to do away with the leg and have a shaft through the boat in long run much more low maintinance. Now the problem I have is that I got a gear box 72 series warner but the ratio is 1:1 Is this going to cause a problem? am i going to lose power?. how do i know what prop to use? do i go for a less pitch prop? do i get bigger blade prop? Or do I get a 2:1 Ratio Gearbox?

[Davor]

New propeller can be calculted, but you should give some data:

Boat : Length on water line
Breadth on wl
Draft (mean)
weight
hull form (V or round)
Tramsom width, draft and area (wetted)
A photo of the boat would be very usefull.

Engine : Power and RPM (max.)

Propeller : Max allowed diameter.
shaft angle (from Water line)

Davor

[Jango]

If you could tell us the MAX. speed (MPH)and Max. RPM with the Out Drive, it is fairly easy to determine Prop size and pitch with the 1:1, shaft drive.

Prop Pitch will be about half as much, since shaft speed is doubled.

[gfinishline]

The only problem with the 'direct drive ratio' is that it loses any torque advantage. I would reconsider your options. You need the maximum torque available at the prop. Reducing pitch is a "bandaid" on your problem. I feel that going under 2.00 to 1 and running a corrected prop pitch will bring the best results. If not, just put a Volvo drive back on the boat. I take exception to the 'junior member' note near my name. I have been designing, building, and rigging powerboats for over 37 years. I am known in the performance areas as a 33 time world champion (powerboat racing), so please change my 'listing' to "old guy who has been there", thank you.

[marshmat]

I take exception to the 'junior member' note near my name. I have been designing, building, and rigging powerboats for over 37 years. I am known in the performance areas as a 33 time world champion (powerboat racing), so please change my 'listing' to "old guy who has been there", thank you.
The forum software assigns those subtitles by default based on your # of posts.... if you click "User CP" (about an inch below the boatdesign.net logo) there will be an option on the bar at left, "edit profile" - you can change it there.

[longliner45]

hope this aint too late for you. how about the volvo duo prop? you can get tractor wheels or speed wheels we had 31ft jc hulls with 6000 lb payloads and twin power units .we ciuld get 20 knots at 1,5 gallons per hr, on slik calm days . only problem is they have a top out of about 40 knots no matter what drives them ...phyisics? ask the engineers on this site good luck

[gfinishline]

I just tried to change it and it didn't go.

[kapnD]

Rich, I recently dealt with a situation similar to the one you describe, and was able to achieve very satisfactory results with the use of a 2:1 reduction remote V-drive while retaining the borg warner 1:1 reverse gear. I was able to slide the fuel tank forward enough to make room for the V-drive and a short jackshaft. A transom mounted rudder was used, in order to get enough clearance for the 20" prop required. The engine had to be turned around, and the mounts were beefed up, and new steering, exhaust and cooling water systems built. It was a lot of work, and really pointed up the beauty of outdrive systems to me! Good Luck!