Reply
 
Thread Tools Search this Thread
 
Old 06 September 2006, 16:30   #1
Member
 
Country: UK - Scotland
Make: Ribcraft
Length: 5m +
Engine: 90hp
Join Date: Aug 2006
Posts: 380
Twin or single for Ribcraft 5.3

Hi,

I'm interested in some opinions of owners with twin engine installations...

Looking at a Ribcraft 5.3. I currently have a 50hp-2stroke, less than a year old, which I was hoping to use. I now have the dilema of whether to use this as a single engine (pretty underpowered for the 5.3) or opt for another 50hp and do a twin install. The transom would need to be lowered for a single install (it's only a long-shaft) so I'm comitted oncee I do that!

I like the idea of twin engines from the safety and security point of view but I realise the overall effective power will be less than 100HP with added fuel consumption... How much extra fuel would I burn with twins? 1.4x?

I was thinking the single plus Aux root but I've had bad experiences with this as the Aux never really gets used properly to keep it running well and I didn't find it that reliable. I guess a 4-stroke Aux (6-8hp) would be a possibility.

Thoughts?
__________________

__________________
al40 is offline   Reply With Quote
Old 06 September 2006, 16:50   #2
Member
 
Country: USA
Town: Oakland CA
Length: 3m +
Join Date: Feb 2005
Posts: 6,653
How much fuel depends on speed, of course. At WOT, probably pretty close to twice, I'd guess. At cruise, well, I suppose it depends on how you're measuring, but for a given speed, I'd guess about 1.5 times (that's a guess, though.)

You would have much more available power, but you're also adding a lot of weight, and increasing lower unit drag by a factor of 2, so the tradeoff may not be as good as you'd think.

Most of the references I've seen say you lose about 1/4 to 1/3 the total rating going with twins (i.e. your twin 50's would net you the equivalent of a 66 to 75 hp single.)

This is all theoretical, as I have never played with twins (I do like the way you can crab the boat around with twins, however. That would make it worth it in my book. Then again, I'm not paying for it, either. )


jky
__________________

__________________
jyasaki is offline   Reply With Quote
Old 06 September 2006, 17:07   #3
mdt
Member
 
Country: UK - Scotland
Town: fife
Make: Humber / searider
Length: 5m +
MMSI: ... - - - ...
Join Date: Sep 2005
Posts: 720
The club have got 2 ribs identical 5.5m bar one with a 75hp 4stroke and one with twin 50hp 4strokes.

The twin picks up quicker from a standstill but hardly night and day over the 75hp one. And there is sod all in the top speed.

Personally I would go for the single given the choice. And as you said get a new-ish 6-8hp as a back up?
__________________
“The only difference between men and boys, is the price and size of their toys”
mdt is offline   Reply With Quote
Old 07 September 2006, 17:47   #4
Member
 
Country: UK - Scotland
Make: Ribcraft
Length: 5m +
Engine: 90hp
Join Date: Aug 2006
Posts: 380
Thanks, usefull info..
__________________
al40 is offline   Reply With Quote
Old 08 September 2006, 16:10   #5
Member
 
Country: UK - Wales
Town: Southampton
Boat name: DynaMoHumm/ SRV/deja
Make: Avon8.4, 5.4 & 4.777
Length: 8m +
Engine: Cat3126 Yam 90 &70
MMSI: 42
Join Date: Dec 2003
Posts: 6,556
theres a weight consideration to putting twins on a shortish rib.

if you alreasdy have the one engine find out it's weight and add the equivalent amount of water in cans to the rear of the boat and see if she rides OK.

I think you get better performance from a single and better economy but twns are kinda nifty if you can afford to run them.

I used to drive a Delta 5.5 with twin yammie 40 2 strokes and I used to take it way off shore but I always thought it would do better with a single 70 or 90
__________________
Here it comes again, I don't stand a chance
Soul possession, Got me in a trance
Pullin' me back to you - Deja Voodoo
Rogue Wave is offline   Reply With Quote
Old 11 September 2006, 17:34   #6
Member
 
Country: UK - Scotland
Make: Ribcraft
Length: 5m +
Engine: 90hp
Join Date: Aug 2006
Posts: 380
Yeah it was the offshore aspect I was thinking about when looking at twins. I had assumed I would be able to plane on 1of2 but that may not be the case. Jury's still out!
__________________

__________________
al40 is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 12:06.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.