Engine HP and Prop Selection???

Advert

New Replies

Report Bug

Help/FAQs

Engine HP and Prop Selection???

This topic is empty.

Viewing 16 posts - 1 through 16 (of 16 total)

Author

Posts
1 June 2010 at 01:17 #27027
Allan Russell
Participant
@allanrussell11393
I’m new to modelling!!! have almost finished the hull of my baby bootlegger 2.24m x 0.45m replica of a 1923 Speedboat. The hull weight is 12kg and is designed to carry 45kg and draw 90mm. Resistance calcs show it reaching the plane at 3.5 Knots with 3.7 newtons total resistance. This seems very little???

I am totally at a loss as to what engine and prop to use, as I can not find a model boat of the same proportions as mine to compare. And I want the thing to get up and boogy!!! Can someone please help?
Advert

1 June 2010 at 01:17 #1970
Allan Russell
Participant
@allanrussell11393
Baby Bootlegger
1 June 2010 at 12:43 #27032
Telstar
Participant
@telstar
Hi Alan As someone that seems to like the numbers.

From info on one version of Baby Bootlegger (off the internet), the full size boat was about 11m and powered by a 300Hp V8 engine.

This would make your model about 1/5 scale (if I can do the math)

So using the scale cubed 300Hp become 300/625, about 1/2 Hp or about 360W

This is at least a starting point for motor selection, I would be looking at brushless motors running on say 2 by 8cell nicads giving about 18v.

Prop selection is a black art I am trying to come to terms with but there are so many different sizes and shapes I wouldn’t like to say.

Hope to see photos of your model on the site

cheers Tom
2 June 2010 at 03:29 #27049
Allan Russell
Participant
@allanrussell11393
Hello Tom, thank’s for your interest. I’m more interested in a petrol engine and have noticed they go up to about 15hp. The price jumps accordingly!!!!! There is one coming out soon according to a website in Australia, that is a 2 cyl, 10hp water cooled unit with clutch for A$680.00, which is excellent value. It apparently revs to 23,000rpm. I wonder if this will be enough to propel the bootlegger at high speed with the right prop?

Cheers Allan
2 June 2010 at 08:58 #27050
Charles Oates
Participant
@charlesoates31738
Hi Allan,

I ought to hesitate before saying that there’s something that’s never been done before, but a model speedboat 2.2 metres long! It’ certainly new to me and probably to most others as well. This might turn out to be one of those models where you will have to do the learning and tell us all what you find out. Having said that some people do build large models and I hope someone will be able to give you some ideas.

It’s a fabulous project, the bootlegers were beautiful looking craft and I look forward to seeing some pictures, especialy when its running.

Charlie
2 June 2010 at 09:45 #27051
Dave Milbourn
Participant
@davemilbourn48782
There’s a dedicated notice board and a thriving membership from the Offshore Model Racing Association (OMRA) on the ModelboatMayhem forum. As your model seems most like the sort of models they race – certainly at the top end of the engine sizes – then I’d suggest posting your question on there to see if anyone can suggest a suitable engine and prop.

I would imagine that 23000 RPM would require gearing down to avoid cavitation.

Dave M
3 June 2010 at 02:12 #27069
Allan Russell
Participant
@allanrussell11393
Thank’s Charlie, I’ll certainly get some photos posted as soon as I can. In fact it may seem a bit of a silly question, but I can’t see how to post photo’s on this site? Can you help please?

Thank you also Dave, I’ll have a look at that website. And I agree that the revs will need stepping down substantially.

I’ll keep you posted on how I go with ModelboatMayhem, Cheers for now Allan.
3 June 2010 at 08:46 #27071
Colin Bishop
Moderator
@colinbishop34627
Allan,

Re posting photos, have a look at the FAQ section – link on the left hand side of the page near the bottom which explains the options. I’m afraid you can’t insert photos directly into a post.

Colin
4 June 2010 at 00:27 #27077
Allan Russell
Participant
@allanrussell11393
Thank’s Colin, cheers Al.
4 June 2010 at 14:32 #27082
Paul T
Participant
@pault84577
Hello Allan

I have been thinking about your engine/prop question and, if it were me I would fit a 75cc 4 stroke Briggs and Stratton petrol engine linked to a home made 1/2in dia shaft and a 4in 3 blade prop.

The 4 stroke will rev slower than a 2 stroke so it won’t need gearing down and it will produce a better exhaust note ( made from copper and water cooled exhaust system)

A unit from a domestic lawn mower would be ideal as it has a clutch already fitted.

75cc is more than enough power to move your boat and the engine will also add a decent lump of ballast to help get the trim right.

Paul
5 June 2010 at 07:58 #27096
Allan Russell
Participant
@allanrussell11393
Hello Paul, thank’s for your input. I’m assuming the Briggs and Stratton is air cooled? of which will pose a problem for my boat, as the motor is fully enclosed. We’re on the same wavelength with the prop though. I feel that a 4 inch prop will be about right if the revs are not excessive. I’ll try and add a picture of my design if I can work out how to do it, Cheers Allan.
5 June 2010 at 15:29 #27100
Paul T
Participant
@pault84577
Hello Allan

Yes the B&S unit is air cooled but can be adapted for water cooling by cutting off the cooling fins with a small grinder (or hacksaw) and then coiling a small bore copper tube around the cylinder jacket.

I would like to see your design

Paul
24 August 2010 at 23:56 #28180
Allan Russell
Participant
@allanrussell11393
Hello All, the Bootlegger is finished except for 2 more coats of clear of which will go on within a week. Photo’s will follow after that and I must say I’m extremely happy with the result. Without the engine and all running gear she weighs 18kg all up and sits in the water exactly where the design predicted. The engine will be the 10hp, 2 cyl RCMK linked to a 3 blade 85mm prop. With everything aboard and allowing for ballast if required, she should weigh less than 30kg, resulting in an estimated top speed of over 100kmh. I got the figures from Dave Gerrs hull speed predictions.

All I need now if anyone can help is some deck fittings! such as cowl vents, fairleads, and cleats. Bearing in mind the boat is 2.4m long, the fittings need to be proportionate in size??? Can anyone suggest where I might start to look for these in brass or stainless please? Kind regards Allan Russell.
1 September 2010 at 12:26 #28224
Allan Russell
Participant
@allanrussell11393
Hello All who may be interested? The Baby Bootlegger is finished and photos are posted.

Engine and running gear next, but have to move house first. Hope you all like the design, cheers Allan Russell.
1 September 2010 at 18:42 #28228
Len Ochiltree
Participant
@lenochiltree67043
Blimey Allan, if you had made it a bit bigger you could have driven it yourself.

A lovely boat build and what a large workshop.

Len.
1 September 2010 at 23:20 #28236
Allan Russell
Participant
@allanrussell11393
Thank’s Len, unfortunately the workshop day’s are coming to an end as we are downsizing in retirement……………I’ve still got my toy though!!! Cheers Allan.
Author

Posts