What's the right Velvet drive ratio??

I know this is a loaded question, but I am looking as a total restore on a 25' Luhrs. I was wondering if I should run either a Chvey 350, or a readily available Chrysler 440. Will the 350 b enough, or would I be better off with the 440? Also what's a good ratio to start with, so all I need to mess with is the prop pitch? Thanks, this is my first inboard boat.

 

All these questions should be put to the propellor man. If you fit any engine then a gearbox and then ask your prop man to make you a prop to make it all work he wont like you very much and may not be able to help you.

Phone your prop man, this is exactly his job. He will make a prop and he will tell you what Hp and ratio to use.

 

Gear ratio and prop sizing has to do with Total Displacement and HP and of course type of Hull - full displacement, semi-displacement, planing etc.

Assuming a planing type Hull, because of size and probable HP, what is the total weight with either the 350 or 440 engine?

 

whatever motor you will be using a 1;1 0r 1;5 to one on that boat, nice gear the Borg Warner
gas in NZ is 7 dollars and 28 cents a gallon, whats it cost by you?

 

Jack, 87 octane (reg) gas out here is about $2.90-$3.15 a gal. I would love to go diesel, because I can get biodiesel for between $2-$2.60 a gal.

As for the weight, I would guess about 6,000 tops fully loaded on a planing hull. I don't think a Chevy 350 would be enough to push her up to 30 mph, but on the other hand, I'm guessing I would be lucky to get 2 miles to a gal at full throttle

 

Not sure of HP, but for 5000+ lbs a 1:1 gear is NOT recommended. Also Prop should be sized at MIN load - driver only and min fuel.

Gear and prop recommendations are as follows:

Assuming 300hp (350) and 5000#min.
gear 1.5:1
prop 16 x 13 (3 blade)
top speed 36.3 mph @ 5000rpm

360hp (440) and 5000#min
gear 1.5:1
prop 16 x 14 (3 blade)
top speed 40.2 mph @ 5000rpm

Speed will be approx. 3 -5 mph slower @ 6000# displacement

 

I Agree lazyjack. Would think 3500 - 4000 lb would be max lite load. No,s would change to:

300HP @ 5000rpm
gear 1.5:1
prop 15x15 4 blade or 16x15 3blade
top speed 43.7 mph

360HP @ 5000rpm
gear 1.5: 1
prop 16x16 3 blade
top speed 47.9 mph

 

Check the strut size to see how big of a prop will fit. Some of the older boats ran 1:1 with small (12"-13") props. This would mean a small strut incapable of turning a 16" prop. Keep in mind, if you replace the strut with a larger one, it will also change the angle of the prop shaft. In most cases, a larger engine will have to work less and therefore last longer and possibly more economical.

 

Good point USCGRET/E8. By going to 4 Blade, you can drop Dia down to 15" if that will help the Strut situation. Also speeding up the Shaft RPM (lower numerical gear ratio) will decrease Dia requirements. Don't recall what is available in Velvet Drive, but 1.25 :1 will also work - 14x13 prop w. 300hp

 

Thanks for the input guys! I will check the existing prop that came with the boat, from what you've told me I'm figuring that would give me an indication of what might have been in the boat originally. As far as the weight goes, my current boat is a Bayliner Saratoga with a peppy 350 in it, and at 4500 rpm (max) and a 15x13 blade, I'm getting 31 mph tops. I weighed the trailer and it came in just about 7,000 lbs, and I got a heavy trailer, so I figured about 5,000 for the boat. This new project has an extra foot of beam, so I guesstimated that I would be weighing about 6,000. Less would be better though!

 

Is your boat similer to this one?
http://cgi.ebay.com/ebaymotors/Luhr...ryZ63726QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

 

Unfortunately I can't get ebay on my stupid gov't computer (I guess they don't want us wasting time LOL) but I'm prety sure I know what boat you're talking about- is it the 25' 1975 Luhrs the guy just rebuilt, all whitw except got he rear area? If that's it, then yes, I'm working with the same exact boat, just older.

 

Bottom Line. If your Minimum Boat weight is less than 6000# a 1.5 to 1 gear should work fine, over 6000#, you should use a 2.0 to 1 gear. This applies to shaft HP,s from 260 to 400, for a planing type Hull. If in doubt go to the higher ratio.

 

Gotcha!