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Old 19 April 2004, 13:54   #1
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Single or twin diesels?

I've run all the numbers, and without a doubt diesel is the way to go for my tour boat. The manufacturer swears that the best motor setup they've installed is twin Mercruiser 1.7L dti's (120hp per motor). This would give me plenty of low-end torque, great maneuverability, and excellent fuel economy. The owners are claiming 15lts per hour for both at cruising speed, about 27mph. My original thought was to go with the Merc D-Tronic 250hp. I've heard nothing but good things about the 1.7's, but not much info on the D-Tronic model.
My problem is that I can't get over the fact that we're talking about 2 120hp motors for a total of 232hp at the props! Merc and Flexboat here both claim that because the motor is designed to operate at higher rpm's, the torque generated compensates for the lower hp rating since I'll be operating only at cruising speed or less. It kinda makes sense, but I thought I'd run it by you folks to get some unbiased feedback.
Two advantages are that there will be less strain on the legs (Alpha 1), which is where many problems seem to occur, and that the D-Tronic costs 4,000 USD more than the 2 1.7's combined. The biggest con is the age-old "twice the motors, twice the headache" theory, but labour is cheaper down here.
Am I far off with my logic? What would you put on this boat?

7.6 mtrs without rear deck
1,900 lbs. w/ no motor or fuel
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Old 19 April 2004, 14:01   #2
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Here's another. And BTW it's rated for 22... 15 in seats, 5 in the bow, 2 crew.
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Old 19 April 2004, 14:04   #3
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If it will really give the performance that you need then it sounds like a good choice as from what you say the money is right and you've got dealer support.

I would definitely want a serious sea trial with this engine setup and a fully loaded boat before committing to anything though!

John
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Old 19 April 2004, 14:50   #4
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Quote:
Originally Posted by John Kennett
I would definitely want a serious sea trial with this engine setup and a fully loaded boat before committing to anything though!
Unfortunately the nearest boat in this configuration is a plane ride away, but I think it's worth for an investment this size.

I've seen a few members post an interest in the twin Merc 1.7 setup, but haven't seen anyone on here that's actually done it. It would be great to find someone who has this setup on these forums.

So I take it no alarm bells are going off with this setup John?

Thanks
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Old 19 April 2004, 15:09   #5
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Hi folks

I belive the 1.7TD 120HP is quite a wide engine, almost as wide as my Yanmar 240HP.

If this is the case, assuming your 7.6M is not much wider then my 7.8 will they fit without staggering ?.

I always though this setup would be very good for cruising, will be very interested if you go this route to see how it works out, but to date I dont know anyone who has done this in a 7M class RIB.

Keep us posted

Regards Gary
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Old 19 April 2004, 15:19   #6
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Quote:
Originally Posted by Garygee
Hi folks

I belive the 1.7TD 120HP is quite a wide engine, almost as wide as my Yanmar 240HP.

If this is the case, assuming your 7.6M is not much wider then my 7.8 will they fit without staggering ?.

I always though this setup would be very good for cruising, will be very interested if you go this route to see how it works out, but to date I dont know anyone who has done this in a 7M class RIB.

Keep us posted

Regards Gary

we have just taken on an order for a Parker 7.5 mtr rib from our Baltic range with twin Mercruiser 1.7 TDI for delivery to a UK customer . This is an interesting project and will keep the forum posted on the built including photos

Andre

PS: Gary now you know
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Old 19 April 2004, 15:58   #7
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I was planning to do this a while ago and width was an issue. Here's a pic of the two engines side by side. It may be of help.
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Old 19 April 2004, 16:33   #8
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I too have entertained this idea. It is a serious contendor on my next RIB list but you guys have put more thought into it. How wide is a yanmar 240, i know putting two of those in a RIB causes a serious problem - like you need to remove one to service it.
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Old 19 April 2004, 16:52   #9
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Quote:
Originally Posted by Daniel
How wide is a yanmar 240,....
699, not including the power steering cylinder.
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Old 19 April 2004, 17:03   #10
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Always a wealth of knowledge. I've voted for you

Given that most RIB's 7m+ have a beam of about 1500mm it would seam one would be having similar problems with a pair of 1.7's as with yanmar 240's. I know which i'd rather have. Don't suppose you have the cost and weight of both to hand anyone?
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Old 19 April 2004, 17:03   #11
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Quote:
Originally Posted by Robbie D
So I take it no alarm bells are going off with this setup John?
I agree with you that it sounds like it's going to be underpowered. However I haven't heard a bad word about these engines and from anecdotal evidence their performance appears to be impressive!

I'd definitely invest in that plane ticket though

John
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Old 19 April 2004, 17:16   #12
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Quote:
Originally Posted by Daniel
Don't suppose you have the cost and weight of both to hand anyone?
1.7 + Alpha drive, 300kgs

Prices vary. 7-10k
---------------------------
Yanmar 260 + Bravo 1, 488kgs

Prices vary.
----------------------------
Yanmar 315 + Bravo 1, 516kgs

Again prices vary, I was quoted about 14K.

-----------------------------------
Always a wealth of knowledge. I've voted for you

Ta.
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Old 19 April 2004, 18:11   #13
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Quote:
Originally Posted by John Kennett

I'd definitely invest in that plane ticket though

John
and make sure it's got a full load when you trial it, put sandbags on if you have to
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Old 20 April 2004, 01:01   #14
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I'll get the exact specs later this week, but as for the engine spacing the 1.7's will hardly be shoe-horned in. The beam on this puppy is 10' 4", with an engine compartment of about 8' across. The engines would be mounted approximately 38" apart on center. A rough estimate gives me about 13" of open space between the motors, so most maintenance can be done with motors in. (You old-timers still remember feet and inches, don't you?)
Without nitpicking, the exact weight of each motor w/ Alpha 1 is 296kg, and the width is 29" or 746mm.
Here's the spec .pdf file if anyone is interested:

http://www.cmdmarine.com/pdfs/18240303.pdf

Thanks for all the responses... keep 'em coming.

p.s. I'm not an engineer, but the torque curve on the spec sheet looks like it would slingshot any 7.5 meter RIB out of the hole with a twin setup, even with 15 pot-bellied tourists.
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Old 20 April 2004, 02:12   #15
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Glad you ran the numbers Robbie and it appears the 1.7's are the way to go
Solent ribs fit these in the 6m up to 7.5 range and they are very good
have not seen a twin set up using these yet but looking at the replies some members are in the process of doing it , as a matter of interest how far is the round trip for a typical charter.
regards tim
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Old 20 April 2004, 03:01   #16
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Quote:
Originally Posted by jwalker
1.7 + Alpha drive, 300kgs

Prices vary. 7-10k
---------------------------
Yanmar 260 + Bravo 1, 488kgs

Prices vary.
----------------------------
Yanmar 315 + Bravo 1, 516kgs

Again prices vary, I was quoted about 14K.

-----------------------------------
Keypart are currently offering the Mercruiser D1.7 (120hp) with an Alpha 1 drive etc for £6450.00.

4 Cylinder Yanmar on Bravo 1 £10,600 available from most dealers at this price!
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Old 20 April 2004, 03:22   #17
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Quote:
Originally Posted by Daniel
...like you need to remove one to service it.
Hmm. This is a concept I'm rapidly becoming familiar with. I more or less have to pull an engine to change the spark plugs on the inboard pair of banks on mine. Having just pulled 1 engine, I've learnt it's not actually a big deal at all though.

This is where I would usually make some quip about mercruiser engines not needing servicing because they are disposable, but in the interest of protecting my rating, I won't.
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Old 20 April 2004, 03:23   #18
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Might be worth calling American Marine Imports up in the midlands - known the bloke for years - might be able to help? 01675 442647
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Old 20 April 2004, 05:10   #19
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Quote:
Originally Posted by Dirk Diggler
4 Cylinder Yanmar on Bravo 1 £10,600 available from most dealers at this price!
B' hell, that's cheap. Perhaps they've been reading the other ribnet thread...or their customers have.
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Old 20 April 2004, 09:33   #20
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Quote:
Originally Posted by tim griffin
Glad you ran the numbers Robbie and it appears the 1.7's are the way to go
Solent ribs fit these in the 6m up to 7.5 range and they are very good
have not seen a twin set up using these yet but looking at the replies some members are in the process of doing it , as a matter of interest how far is the round trip for a typical charter.
regards tim
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Tim, the round is relatively short, about 8 to 10 NM depending on the route. Speed will vary a lot... cruise, slow moving around rock walls, and full stop for swim time. Might see some extra rpm if we spot a whale or dolphin pod. Unless it's a charter the average round will only be about 1:15 minutes.

One thing I find interesting in this thread is that I haven't seen the usual beef about 2 engines being purely for redundancy, as is the case for petrol outboards. Is it possible that this configuration might actually increase performance? Come to think of it, I think this was my original question for this thread...
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