Where to start help.

bimmi

Cadet
Joined
Oct 31, 2013
Messages
9
I;m building a donzi hull 20 ft 2900 lbs engine is 383 425 hp 450 torque volvo drive 1.61 ratio 5400 wot. want to use 4 blade ss I care more about top end . would this engine pull a 25 pitch 4 blade 70 mph? thanks.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Where to start help.

As far as I know if your looking for top end a 4 blade wouldn't be the choice.There are others that can tell better what will work best.
I would think it could turn a 25" prop.
5400 1.61,25",12% slip = 69.9 mph.
 

Starcraft5834

Lieutenant Commander
Joined
Jun 2, 2013
Messages
1,677
Re: Where to start help.

A 4 blade will toss your boat on plane much faster,, 2900lb? keep the 4 blade on it... you might lose 2mph speed comparred to 3 blade...

Ive got a 20ft cuddy, had a 3 blade, went to 4 blade, performance difference was like night and day.........relating to hole shot. losing 2mph,, I could care less... 4 blade steered much smoother at trolling speeds also......a noticeable difference for sure...
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
71,264
Re: Where to start help.

Ayuh,... With that kinda power, 'n wantin' to go Fast,...

I'd think the 1st thing to do is get a 1.50:1 gear ratio in the drive,...

What drive is it,..??
 

bimmi

Cadet
Joined
Oct 31, 2013
Messages
9
Re: Where to start help.

Ayuh,... With that kinda power, 'n wantin' to go Fast,...

I'd think the 1st thing to do is get a 1.50:1 gear ratio in the drive,...

What drive is it,..??

volvo penta corba what would changing ratio do. could;nt you off set ratio by increasing prop pitch? I don;t know. thanks
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Where to start help.

volvo penta corba what would changing ratio do. could;nt you off set ratio by increasing prop pitch? I don;t know. thanks
Trying to make the wrong gear ratio work by a prop selection seldom works.
Bondo knows his stuff.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
71,264
Re: Where to start help.

volvo penta corba what would changing ratio do. could;nt you off set ratio by increasing prop pitch? I don;t know. thanks

Ayuh,... Generally speakin', changin' the ratio does 2 things,...

#1, it puts the prop rpms into an Efficient range,...

#2, props are generally available from 15" to 23" pitches,...
Get outa that range, 'n props become rare, 'n expensive,...

If ya wanta do it Yer way,...
Put it together, slap a prop on it, 'n Try it,...
Atleast you'll have some numbers to work with,...
 
Last edited:

dan02gt

Chief Petty Officer
Joined
Aug 30, 2012
Messages
463
Re: Where to start help.

I would recommend a Mercury Bravo 1 for your application. They make them from 22-36 pitch. Make sure to order from someone that will let you exchange it if the pitch is incorrect. I would think you could turn a 25 but only testing will tell.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
71,264
Re: Where to start help.

bimmi said:
bondo still trying to grasp out drive ratios why different ratios to start with. you say 1.51 is more effecient then 1.61 i;ts only 6%lower so if i go to say 26 pitch at1.61 v.s. 25 pitch at1.51 isn;t final drive close to same ? thanks i,ve been around race cars all my life but performace boating hole knew ball game with prop slip cupping ect.

Ayuh,.... Have ya looked for, found, 'n priced props in the ranges of pitch yer lookin' at,..??

As I said,....
It can be done yer way, with props, but if 30" pitch props were Better, We'd all be runnin' lower gear ratios, 'n taller props,...

If this is an off-shore style hull, 'n yer Hot-roddin' it, ya might wanta check out some forums that deal with those sorta hulls,...
Off-shore only forums, I think caters to that crowd,...

Have ya run the hull,..??
Does it need more bow lift, or stern lift,..??
 

jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
Re: Where to start help.

What is a "Volvo Penta Cobra"?

Some preliminary calculations indicate that you might be able to turn a 21-23 pitch at about 5200-5300 rpm @ 57-58 mph.

Seventy mph with that combination might be a pipe dream. :faint2:
 

bimmi

Cadet
Joined
Oct 31, 2013
Messages
9
Re: Where to start help.

98 was last year of take over of omc by volvo penta i really don;t know what your cal. the 20 ft boat 2900 lbs with the old 305 19 pitch would run right 52mph so i;m thinking adding 150 to 175 hp to around 425 to it should swing a25 to 26 wheel all day thanks for your cal.
 

bimmi

Cadet
Joined
Oct 31, 2013
Messages
9
Re: Where to start help.

so what your telling me is you do not have a clue AYUH.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Where to start help.

Probably not the smartest thing to drive away a guy with 50 thousand posts.
Seems he might have a "clue"
And a guy with 5 posts may not.
 

Starcraft5834

Lieutenant Commander
Joined
Jun 2, 2013
Messages
1,677
Re: Where to start help.

agreed steel,, this forum has been of great help to me personally,, I pay attention to folks that have been down the road before.. I have learned much in 1 season and look forward to learning more... you, Bondo and Achris to name a few have all been very helpful....this forum saved me lots of money........
 

Tail_Gunner

Admiral
Joined
Jan 13, 2006
Messages
6,237
Re: Where to start help.

You will be very close to your goal if everything lines up right...Going to a smaller gearing will help you with your hole shot and your top end...the smalller the pitch the less drag (prop) you will have.What you really have going on is huge tourqe on a light boat...Solas 4 blade SS are known performer's on Donzi's...the Turbo FXP in a 3 blade is also known as a top performing prop on a Donzi..Dont let the info on those props throw you..I a turbo 0T4 FXP iam playing with now it slips at 8% on a 22' 4500 lb deep v...that's stellar. Just need to get the engine working as well as that prop...

Get your hands on a 24p..could be 23...seriuously doubt a 25...anytime the tourqe get's that large on a light..short boat it's hard to work out.
 
Last edited:

bimmi

Cadet
Joined
Oct 31, 2013
Messages
9
Re: Where to start help.

Tail gunner thank you for your advice finally something I can work with i:ve had some drive by mechanic,s with vague overview,s you know the type the sky is blue water is wet. repeating the same thing 50,000 times still does,nt make it true they probably voted for obama. bondo you can go back to reading this month,s Barge and paddle"
 

Tail_Gunner

Admiral
Joined
Jan 13, 2006
Messages
6,237
Re: Where to start help.

Tail gunner thank you for your advice finally something I can work with i:ve had some drive by mechanic,s with vague overview,s you know the type the sky is blue water is wet. repeating the same thing 50,000 times still does,nt make it true they probably voted for obama. bondo you can go back to reading this month,s Barge and paddle"

Easy Bimmi I like to hotrod boat's but would take a back seat to Bond-o 7 days a week. What Bondo is talking about is a basic fundamental of hot rodding. Using the correct gears will improve your hole shot quite a bit and increase you topend performance.. Welcome aboard give it sometime...the inmate's and quite knowledgeable.
 
Top