Page 1 of 2

Let's be silly or maybe not!

Posted: Sun Dec 10, 2017 12:49 pm
by Steven r
Hi all,
How fast will a 30-33 feet 7.6 beam boat go? With 850hp with 1450 of torque 2500 rpm it needs to carry a fair amount of petrol in the sturn? Gearbox no idea at the moment, how much hull thickness? Iam I mad!!
Steve.

Re: Let's be silly or maybe not!

Posted: Sun Dec 10, 2017 1:32 pm
by Bill Edmundson
Steve

Without a lot more info, I don't have a good Idea. I could make a good tugboat, limited to hull speed at 8.5 mph. Or, a trawler at about 18-20 mph. In planning hull maybe 25 to 27 mph. Gearing and prop are going to be a trick. It will be hard to get enough shaft rpm to get much speed.

Bill

Re: Let's be silly or maybe not!

Posted: Sun Dec 10, 2017 2:32 pm
by Steven r
Bill,
A runabout! A stretched miss apba, looking at the 40ft that is being built in Canada. Hacker built for Henry Ford a 33ft with a libity 450 Hp v12 in 1924 they said it did over 60 mph. So how about a Rolls Royce Metour? Up skaleing the plan to 30-33ft. I think I am going mad!
Steve.

Re: Let's be silly or maybe not!

Posted: Sun Dec 10, 2017 7:29 pm
by kens
as boat size goes up, shaft rpm does not go up as in smaller boats. at least not unless you are going 100mph.
Example: the mahogany's at the G might turn a 13x13 and run 30mph with a V8 1:1 gear
A big sportfisherman might be 45', run a honkin' diesel with a reduction gear, and turn a 30x30 prop. and still make 30mph.
the shaft speed might be 1/3 of the mahogany, but the pitch is 3x

the boats with a long length per beam run really well, and get the best use of power,
it is not silly to see a beam/length ratio of 4:1 get up on plane and move out.
saying you want to use a merlin, is, silly.
if you had a 8' beam and a length of 32', then that is 4:1. if you had a modern diesel in there you could make it go

here, build this:
http://gerrmarine.com/HM_POPE_III.html

Re: Let's be silly or maybe not!

Posted: Mon Dec 11, 2017 11:01 am
by Steven r
Kens,
I have a semidisplayment boat Nelson 34x9.0 feet with 2x250hp sabre diesels it goes really well 30 knots wights8tons and has 1.5.1 boxes 24x24 props and revs to 2450 rpms. We have had her 30 years! I just fancy a maghony for when we move to the sun.
I want something like Evangeline but it's to rich!! There are some engines in packing cases just under 10k plus tax in the UK,(Rolls Royce) but the put off is the 100gph it could burn but at 40_50mph it may not be that bad? Petrol is very expensive here!
Steve.

Re: Let's be silly or maybe not!

Posted: Wed Dec 13, 2017 11:36 am
by Steven r
Hi,
If you have a 30 ft runabout with 7feet beam Wight 2.5 tons with 1.1 gear ratio 2500 rpms prop 21 x39 pitch 4 blades any idea how fast it should go?
Thanks Steve.

Re: Let's be silly or maybe not!

Posted: Wed Dec 13, 2017 5:08 pm
by kens
you must define available horsepower to get speed.
speed is a function of HP vs weight

Re: Let's be silly or maybe not!

Posted: Wed Dec 13, 2017 5:50 pm
by Steven r
Kens,
Sorry missed that out 850 Hp and torqu is1450 and gearbox 2 speed maybe?
Thanks Steve.

Re: Let's be silly or maybe not!

Posted: Wed Dec 13, 2017 7:41 pm
by kens
looking at a LBS/HP ratio of 2.5ton per 850hp, I'll open the betting at 60mph.
if you say 2500rpm then I'll bet 39 pitch is over pitch by 14 inches

Re: Let's be silly or maybe not!

Posted: Thu Dec 14, 2017 12:40 am
by Steven r
Kens,
I was hoping it would go a bit faster than that!! :lol:
What do you dislike about this engine? I like American power, we have Lincoin navigator, Ford F 250 diesel, and a Jensen Interceptor that has American power! That I used to drive in the 70s!
I can think other engines mopar 440 or 454 but the RR would look right for the type of boat.
Thanks Steve.

Re: Let's be silly or maybe not!

Posted: Thu Dec 14, 2017 9:59 am
by Jimbob
Guess I will chime in.
If you want to go fast, I would suggest a offshore race boat. (cigarette boat). Remember the Wellcraft Scarab boat in Miami Vice?
There are two glen l designs that might fit the ticket, The Bandido and the Slot Machine. If you want to go really fast, equip them with twin 502 mercruser i/o's. I would suggest a boat with the deep V hull, because you want to be safe at high speed. I had a 20' Wellcraft deep v hull with a 5.7L i/o that would do 70. Hull design has a lot to do with speed.
Jim
(Building a slow cruiser now.)

Re: Let's be silly or maybe not!

Posted: Thu Dec 14, 2017 10:28 am
by Steven r
Hi Jim,
First may I say your boat is coming on very nicely!!
Jim I want what can't have ha ha :lol: it's 33 ft built buy Hacker, Evangeline with a libity engine 450hp they say it does over 60 mph. I have some ideas so what this space!
Regards Steve.

Re: Let's be silly or maybe not!

Posted: Thu Dec 14, 2017 10:44 am
by Jimbob
Here's one to consider

Lake Tahoe's Thunderbird
Has twin V-12 Allison Aircraft Engines
1100 hp each (max about 3300 hp each)
Turns twin 22" Props
Watch the startup below!

https://www.youtube.com/watch?v=r9DGUItTamI

Good video of the boat running on the lake.

https://www.youtube.com/watch?v=JlutXubHdlI

Jim

Re: Let's be silly or maybe not!

Posted: Thu Dec 14, 2017 4:51 pm
by kens
Jimbob wrote:Guess I will chime in.
If you want to go fast, I would suggest a offshore race boat. (cigarette boat). Remember the Wellcraft Scarab boat in Miami Vice?
There are two glen l designs that might fit the ticket, The Bandido and the Slot Machine. If you want to go really fast, equip them with twin 502 mercruser i/o's. I would suggest a boat with the deep V hull, because you want to be safe at high speed. I had a 20' Wellcraft deep v hull with a 5.7L i/o that would do 70. Hull design has a lot to do with speed.
Jim
(Building a slow cruiser now.)
no,
this is the boat for the same outcome:

http://www.boatdesigns.com/30-Bandido-d ... ducts/197/

Re: Let's be silly or maybe not!

Posted: Thu Dec 14, 2017 4:58 pm
by kens
Steven r wrote:Kens,
I was hoping it would go a bit faster than that!! :lol:
What do you dislike about this engine? I like American power, we have Lincoin navigator, Ford F 250 diesel, and a Jensen Interceptor that has American power! That I used to drive in the 70s!
I can think other engines mopar 440 or 454 but the RR would look right for the type of boat.
Thanks Steve.
Not that I dislike it, but rather, how would you marinize it?, how would you maintain parts for it? how much would it drink fuel?
By the time you get that marinized, you could probably buy 2 (two) big block Chevys, with equal or lighter weight, better fuel consumption, parts availability, and comes with a factory warranty.
If you want to talk silly, take 2 BBC's and turn them facing each other, (2/3 of a PT boat) and put them in your 7' beam. !!!!
Do that, and I bet you beat 60mph. !!!