Reply
 
Thread Tools Search this Thread
 
Old 10 January 2011, 20:38   #1
Member
 
Country: UK - England
Town: Manchester
Boat name: n/a
Make: n/a
Length: under 3m
Engine: n/a
Join Date: Apr 2009
Posts: 76
Are two twice as good as one?

I haven't posted any of my "Gordon Bennet, I thought everyone knew that" questions for a while, so here goes:

I can understand that if you run a small boat, such as 3 or 4 metre, or you only use it on rivers, lakes or very close inshore, having a twin engine installation may be a little over the top. Likewise if you have a 7 or 8 metre vessel and sail a long way from shore then a twin motor set up would probably be a really good idea.

But, bearing in mind that two 75hp engines probably weigh more than one 150hp, or two 90s are almost certainly heavier than a single 175, are there any advantages to having a twin set up?
__________________

__________________
The Beard is offline   Reply With Quote
Old 11 January 2011, 03:12   #2
Member
 
chewy's Avatar
 
Country: UK - England
Town: Up Norf
Make: Avon SR4,Tremlett 23
Length: 4m +
Engine: Yam 55, Volvo 200
Join Date: Apr 2007
Posts: 5,217
I'll say it before anyone else does, have a quick search and you'll find your answer.

Twins won't put out the same hp as a single, you lose something like 30% using twins, Ralph has formulas for drag etc.
Twins are good if you have two completely independent systems, fuel batteries etc.
You've got twice the running costs and servicing costs....

Quote:
Originally Posted by The Beard View Post
But, bearing in mind that two 75hp engines probably weigh more than one 150hp, or two 90s are almost certainly heavier than a single 175, are there any advantages to having a twin set up?
I know its just an example above but the Yam 75 two strokes are the same as the 90, so twin 75's would weigh the same as twin 90's.
__________________

__________________
chewy is offline   Reply With Quote
Old 11 January 2011, 04:38   #3
Member
 
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,611
Yep, the "twin X is heavier than a single 2X" is 50% b*llox. As chewy points out, most manufacturers use the same lump of hardware for two or three horsepowers, and two yam Xs may be heavier than a Yam 2X, but the Evenrude twin Xs may be light than the single. You need to look at each setup individually. (and remember the weight of the aux as well!)

The difference becomes more pronounced at bigger Hps because speeds (& therefore drag) goes up, and more Hps tend to share the same lump of metal.


Search for the previous threads, I have posted worked examples to illustrate that summary a bit better......
__________________
9D280 is offline   Reply With Quote
Old 21 January 2011, 21:12   #4
Member
 
Country: UK - England
Town: Manchester
Boat name: n/a
Make: n/a
Length: under 3m
Engine: n/a
Join Date: Apr 2009
Posts: 76
Thanks both. I should have tried the search function. Good points about the weights of some motors being the same as more powerful versions. I looked on the Evinrude site and, sure enough a 75 weighs the same as a 90 and a 150 and a 175 are also the same. Thanks for pointing me in the right direction though.

Sometimes what seems a mystery to a novice is blindingly obvious to those with more experience.
__________________
The Beard is offline   Reply With Quote
Old 22 January 2011, 07:31   #5
Member
 
Channel Ribs's Avatar
 
Country: UK - Channel Islands
Town: Alderney
Length: no boat
Join Date: Sep 2006
Posts: 3,047
It is a good question and it tends to become a marmite issue, some people love twins and would not want to be without others feel just as strongly about single engine installations.

Twins are great if you have unlimited cash or time for servicing, as Chewy says you also need to have totally separated systems in order to get any real benefit so your looking at twice the wiring/hoses.

Rescue boats often have twins, you get a bit of extra maneuverability and the redundancy is an obvious benefit. But resale values can suffer, most people want a cheap to run setup.

You can also get a false sense of security, believing that your boat will perform just as well with one of the two engines still working. This is only true in high end rigs (Martini's springs to mind).
__________________
Channel Ribs is offline   Reply With Quote
Old 22 January 2011, 07:59   #6
Member
 
Country: Canada
Town: Tobermory, Canada eh
Boat name: Verius
Make: Zodiac Hurricane 590
Length: 5m +
Engine: Yamaha F150
Join Date: Oct 2006
Posts: 1,345
Send a message via MSN to Stoo
But, but, but...

Twins SOUND so cool and look awesome....
__________________
Pump it up and RIDE!

www.wetspotimages.com
Stoo is offline   Reply With Quote
Old 24 January 2011, 05:25   #7
Member
 
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,611
Quote:
Originally Posted by malthouse View Post
as Chewy says you also need to have totally separated systems in order to get any real benefit so your looking at twice the wiring/hoses.
Although at the smaller end of the scale, that only really applies to fuel, as most "small" engines are self sustaining once running and "pull start-able", so the whole twin battery / wiring argument is slightly irrelevant, and below about 50/60Hp, usually run from portable tanks anyway. Bottom line, can I start / run it without a battery?


Quote:
Originally Posted by malthouse View Post
Rescue boats often have twins, you get a bit of extra maneuverability and the redundancy is an obvious benefit.
...and on "calm" days you can pootle about on one and half the fuel bill...... (I am taking sailing club type rescue here, not RNLI out at 3am in a F9!)


Quote:
Originally Posted by Stoo View Post
Twins SOUND so cool and look awesome....
Yeah, the thought that ambled round my mind as I rebuilt the Clamshell..... two of the old Merc straight sixes...... or two of the 4 cyl 30s for a slightly less thirsty setup
__________________
9D280 is offline   Reply With Quote
Old 24 January 2011, 05:29   #8
Member
 
Channel Ribs's Avatar
 
Country: UK - Channel Islands
Town: Alderney
Length: no boat
Join Date: Sep 2006
Posts: 3,047
Quote:
Originally Posted by 9D280 View Post
Bottom line, can I start / run it without a battery?
Not usually the case with 75s or 90s.
__________________
Channel Ribs is offline   Reply With Quote
Old 24 January 2011, 05:35   #9
Member
 
chewy's Avatar
 
Country: UK - England
Town: Up Norf
Make: Avon SR4,Tremlett 23
Length: 4m +
Engine: Yam 55, Volvo 200
Join Date: Apr 2007
Posts: 5,217
Quote:
Originally Posted by malthouse View Post
Not usually the case with 75s or 90s.
We used to pull start the Evinrude 70's a piece of p!ss, haven't tried the Yam 75's although if you can start a 75 and the 90 is usuing the same block it shouldn't be a problem?
__________________
chewy is offline   Reply With Quote
Old 24 January 2011, 05:39   #10
Member
 
Channel Ribs's Avatar
 
Country: UK - Channel Islands
Town: Alderney
Length: no boat
Join Date: Sep 2006
Posts: 3,047
Quote:
Originally Posted by chewy View Post
We used to pull start the Evinrude 70's a piece of p!ss, haven't tried the Yam 75's although if you can start a 75 and the 90 is usuing the same block it shouldn't be a problem?
I would still prefer to have switchable batts and use the mighty key.
__________________

__________________
Channel Ribs 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 17:54.


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