Help me make the decision ... twin or single diesel?

[piperca]

I am getting close to getting back into my boat project, since my home remodel is getting close to completion ... :)

I have a 28' Skipjack Flybridge that I have removed the twin gas engines from and am now considering repowering with diesel(s). My dillema is twins or a single. I am considering either a pair of rebuilt Volvo Penta AQ31P-As (one is actually new) or a single rebuilt Yanmar 6LP. The Volvos are 150 hp each and the Yanmar is 315 hp. Price to install either is not an issue, since it works out close to the same ... couple of thousand more for the single.

Yes, it would be a lot easier to drop twins into the current setup and use the existing outdrives, but I'm looking at maintenance, also.

There's a bunch of transom patching and stringer reconfiguration to take place if I do the single. I'm a carpenter by trade, so that doesn't really scare me too much, I'm just a little burned out with remodeling projects, I guess.

Anyway, given the options, those of you that have experience with this type of work; which way would you go? Why/why not?

I have a terrible time making up my mind, so anything you guys can throw at me would help immensly!

[El Sea]

FWIW:
Since I favor the single engine concept, I would go the Yanmar, this would be a good time to get those stringers and transom in shape.

If you were determined to go twin, I would go a pair of smaller Yanmars, I really don't get a true, warm fuzzy feeling when it comes to Volvo's.

Luther Carrier
Absolute Tank Cleaning
St Petersburg, Fl

[piperca]

Thanks for the response!

Do you have experience repowering from twins to a single? I am wondering how this would affect the boat, since it was designed to be run with twins? The guy who is going to do the install thinks that a single is a bad idea; he thinks it will affect the boats center of gravity in a negative manner, since it is an 8' beam and is tender on the drift with twins ... comments?

[El Sea]

I would hope that when the twins are removed along with the shafts, struts, etc a keel would be layed to give stability and protect the prop/shaft.

Keep in mind with a 8' beam and twins, how are you going to maintain them?

[piperca]

Sorry, I should have clarified; the boat had twin I/Os (sterndrives), so there's two lovely big holes in the transom that will need filled! Here's a pic of what I was left with:

http://i275.photobucket.com/albums/jj316/piperca/P4060004.jpg

[mydauphin]

The boat was design for twin stern drives gas engines. The new diesel engines will probably be a lot heavier (first Problem). and if you don't use stern drives what other type of drive are you going to use. You don't have room for much.

I would go with single diesel, surface drive.... Save a alot of work with rudders etc... Check out Levi drives and you dont have to open holes in bottom.
I had a 8 foot beam with twin engines, what a maintenance nightmare.

[piperca]

Yes, weight was an issue, but I decided on Volvo Penta 31s, which are very close to the weight of the small block Chevys. Now, if I were to go with a single, I'd be dropping close to 1000lbs ... nice thought!

Surface drive? I am not familiar with this type of drive; heard of it, but know nothing about it. Can someone guide me to where I might learn a bit about them (installation, maintenance, etc.)?

And, yes, I have learned that the twins are a maintenance nightmare and would like to leave that behind!

BTW: I was just looking over some information on these surface drives. How do these drives perform in a fishing environment? Do they become a bit of an obstacle when fighting a fish? They appear to hang pretty far off the stern ... am I wrong about this?

Update: I don't think the surface drive will work for me. According to what I've read, they are not preferred for slow speed, like the speeds during trolling, etc.

[CDK]

The difference in weight between a small diesel and a gas engine is no issue, certainly not when you contemplate changing from stern drives to surface props.

Admittedly, maintenance on 2 engines takes more money & time than one, but having two engines makes a lot of difference in handling your boat in narrow spaces, crosswinds etc. And in case of a defect or prop damage, it can make the difference between getting home by yourself or waiting for a tow.

I changed from stern drives to jets, then after 2 years of jet misery to tunnel drives. Took lots of time and resources, but it was very rewarding.

[Frosty]

That looks pretty tight for two Yanmar 6LP's.

There is a big intercooler on the port side and a big turbo on the Stbd.

Twins are better but you need a tape measure.

Yanmar 6Lp are good engine --high reving for a surface drive but then again your engine room is tight in so much as length with a gearbox.

If you wait a couple of days I can give you all dimensions of the Yanmars ,--I have a pair.

[Submarine Tom]

Piperca,

I don't envy your dicision position.

Pull out a piece of paper and list the pro's and con's on each side and then

give them an importance value of 1-10. This may help give you a

numerical summary of your desires.

Have you considered outboards? Like you need another option...

How important is weight to you?

What would you do with the extra bilge space?

Some other considerations may be:

resale value, ride comfort and noise level, stability (sea worthiness),

length overall, cost (install and maintenance), time out of service,

what the wife wants, and probably several other things I can't think of right

now.

Good luck! Let us know what you decide.

Tom

[daiquiri]

Just one shallow question - why do you want to switch to diesels? Diesel engines are an excellent choice if you spend a considerable amount of time at sea. In that case the overall fuel cost savings can compensate for the bigger cost of diesel engines in, say, 4-5 yrs of use. So the first thing to do is an estimate of number of engine operating hours each year.
In case you already did this math and decided that diesels are the way to go, than you have to take care of several things over there:
- weight of new engines vs. the old ones. You have a planing boat over there and the weight distribution is an important point.
- dimension of new engines vs. available space.
- don't forget to leave enough space for a mechanic to visualy control and insert his hands between the engines and do whatever he needs to do. You shouldn't leave less than 30 cm (12") of spacing.
P.S. - no, surface drives are not what you need if you are looking for good low-speed and trolling capabilities.

[piperca]

I weighed the gas/diesel dilemma and this is what I found:

-Gas is about 7k a side (Marine Power w/closed cooling).

-A couple of low hour diesels would cost me about the same and I could use my existing drives (280s).

-A single Yanmar is around 12K; outdrive, another 5K.

-I installed new fuel tanks, which have never seen fuel.

-No GAS under my aft berth to blow up my wife and kids.

I've got one BRAND NEW Volvo Penta 31P-A (4 cylinder, 150hp), but haven't been able to find a match ... this would be the easiest option. I'd snug these up to my existing Volvo Penta 280s and call it done!

The Yanmar would reduce weight, reduce overall maintenance (single vs twins), reduce fuel burn, but I'd sacrifice maneuverability. I would probably see better performance from the Yanmar than the twin Volvos, due to the drop in weight and higher horsepower rating of the Yanmar. I'd need to invest in a newer outdrive (Bravo 3x or equivalent), which will set me back a few more dollars.

So, I think I've settled the gas versus diesel dilemma regarding the difference in cost to repower; therefore, I can shelf that.

Now, back to the original question; twins or single?

The Volvos are no bigger than a gas 350, so I know I'll have plenty of room after installation. In fact, I've seen a lot of Skipjacks with twin Volvo 41s installed (6 cylinder), which is the choice setup for this boat, so I'm not concerned.

I guess maintenance and maneuverability appear to be the main issues, which gives a knock to each category, single and twins ... which can I live with???

Thoughts? Am I missing anything?

[Submarine Tom]

"Now, back to the original question, twins or diesel."

Diesel.

[piperca]

"Now, back to the original question, twins or diesel."

Diesel.

Got me! :p

[Jango]

Been following the discusion and based on what I've heard, I would go for Twin 150HP diesels.

[mydauphin]

If you are going to keep boat....Diesels, Make sure you use closed cooling and route exhaust properly to prevent salt water flowing back into engine.

[piperca]

Been following the discusion and based on what I've heard, I would go for Twin 150HP diesels.

That's the way I've been leaning, but the nearest rebuilt I can find to complement the new one is in Vancouver, Canada! I guess I'll just be patient and see what turns up. I've got another month or so before the house remodel is complete.

[big-boss]

What did you end up with? I am in the same deliema but with a 27ft Uniflite and I have two ASD-6 Surface drives.

[piperca]

I'm leaning towards the twin Volvo four bangers, but I've got to find a second engine that will meet my needs. I think I'll be sticking with Volvo 280 drives, since all I've read regarding surface drives does not fit my application (fishing, trolling, etc.).

[piperca]

Okay, I may have found that second engine to complete the twins setup ... yeah!

Minor problem: My existing drives are VP 280s with a V8 gear ratio (1.61:1, not sure). Anyway, does anyone know what ratio I should be running? If I were to stick with the existing drives, what issues would I have?

Thanks in advance!

[Karl2]

Piperca,

1.61:1 is the ratio to run with the 280 drive. The 150 hp 31 was never offered on the 280/290 style single prop drive. The last version was predominantly sold with the Duoprop drive and then with a 2.3:1 ratio (Don't compare ratios between duoprop and single prop drives). The engine was offered with the "SX" single prop drive (The drive used on Volvos gas motors) and then with a 1.79:1 ratio.

Looking at it strictly from an effiency standpoint you would be better off with a 1.8 - 1.9:1 ratio but you would then sacrifice gear life - Don't go there.
If you are looking for efficiency (And several other performance related benifits) spend your money on Duoprop lower units, look for used ones with the 1.95:1 ratio (The 2.3:1 ratio requires a complete drive - this ratio is made up with a reduction in both the lower and upper gear case). Or, if you have the apetite and the $; I believe Volvo has an exchange program were you give them your single prop lowers and they will sell you duoprop lowers.

Again, If you stay with the 280 units stick with the 1.61:1

Good luck

Karl2

[piperca]

Thanks, Karl2!

One of the engines I will be using is new and has attached a HS1 gear in the 2.62:1 ratio; that made me think I was way out of sync. Obviously, there is a bit of a difference in ratios; what is the greatest difference I would see between the two?

[Karl2]

I'm not sure I understand your concern. Both engines will be coupled to a 280 drive with a 1.61 ratio. The HS1 gear with the 2.62 ratio will be in your garage or sold off to someone.

Karl2

[piperca]

I'm not sure I understand your concern. Both engines will be coupled to a 280 drive with a 1.61 ratio. The HS1 gear with the 2.62 ratio will be in your garage or sold off to someone.

What I meant is what performance related issues would be seen running the lower 1.61:1 gear ratio in comparison to a higher gear ratio, as in the ratio of the gear that accompanied the engine. Obviously, that gear ratio was coupled with that engine for a reason or am I missing something? Sorry, I'm a complete amature in regards to this topic. :o

[Karl2]

OK - Understand.

When I say that perhaps a more ideal ratio (from an efficiency standpoint) would be a 1.8 - 1.9:1 I speculate that such a ratio would be a better match for the speed range that this engine/boat combination will result in. A 1.8 - 1.9 ratio could result in a small top speed increase and probarbly better acceleration. This would be small differences and nothing you should loose any sleep over.

The 2.6:1 gear that the engine came with is a bit unusual. Most of these motors in the in-board configuration were sold with a 2.0:1 gear. The 2.6 was probarbly destined for a bigger/heavier/slower boat were you need to swing a bigger diameter prop to get decent efficiency.

Again, in your case you will be just fine with the 1.61:1.

Karl2

[piperca]

Excellent, Karl2, I really appreciate your input!

One last question, if you don't mind. What would you recommend as a starting prop size in this configuration? The boat is a 28' Skipjack Flybridge, which has a dry weight with engines of approximately 6600lbs. I am hoping to gain a speed of around 18 knots cruise. The prior setup (twin AQ225Ds, 225hp 305s) I was getting around 16 knots cruise; I was running 15x17s. I have a pair of 15x19, but I feel they might be a bit under sized ... what do you think?

[Karl2]

What was your top speed with the 225's and at what rpm ?

Karl2

[piperca]

What was your top speed with the 225's and at what rpm ?

Karl2

I don't really know what my top speed was, since I never had any reason to run her that hard. The cruise was 15-16 knots (18-19 mph) @ 3100-3200 rpm. WOT on that engine is 4400. I had her at 4200 once (momentarily), to check that the engine was propped correctly. I didn't want to max her out in the sea conditions at the time and was happy with the 4200, since I still had a little throttle left ... that was running 15x17s. I failed to check the speed at that time ... stupid Andrew!

[Jango]

According to Prop Calc your props should be 17 x 16,s. Larger props are requires due to the slower running motors - 3600 RPM Max. w.1.61 gears. Top speed at 7600 lb disp. should be around 24 knots, with a cruise of 15 knots @ 2500 RPM.

[Karl2]

OK – I’m going to guess that the boat is a bit heavier than 7,600. My best estimate is a top speed around 24-25 knots. Prop should be selected so you can get 3,900 @ WOT (These engines are NOT 3,600 rpm) with a loaded boat. If you achieve 24 knots at 3,900 rpm you can, safely, cruise all day @ 3,400 – 3,500 @ 20+ knots. So..Given that the above pans out your 18 knot cruise target should be there and at a very fuel efficient rpm.

As to the prop: Jango is correct – This combination (Engine hp and rpm/gear ratio/boat/speed range) would work better with a larger diameter wheel. However 17” is not available and would not fit.
Volvo has 16” diameter props. My calculation falls right between a 16x15 and a 16x17. My recommendation = 16x15.

Karl2

[piperca]

Thanks Jango/Karl2!

Okay, another problem just arose; the engine I was looking at turns out to be an AD31B and not a match to the 31P-A that I have. The 31B is a 130hp engine and the P-A is a 150. Is it possible to tune the 31B to match the performance of the 31P-A for a twins setup? I am having a terrible time trying to track down another P-A!

[piperca]

Karl2, I calculated out, approximately, what the boat would weigh fully loaded.

I figured 6600lbs dry, 1200lbs fuel, 800lbs for 4 persons, 500lbs gear, 600lbs bait tank and fresh water tank. Looks like she's running not a kick in the butt off 10,000lbs. Would this affect the calculation for the props?

Of course, when I'm out with the family on a cruise, she'll be close to 1000lbs lighter, since I probably will not be fishing.

[Jango]

Very little effect on Props - Calculates to 16x14 Props @ 3900 motor rpm with 1.6 gear, 9000# total, resulting in 24 Knots w. 15% slip. Cruise should be at least 14 - 15 knots @ 2600 rpm ( abt. 24% slip)

(You may get by with 16x15,s but 16x14,s are ideal with 300 max HP and the heavier load)

[piperca]

Very little effect on Props - Calculates to 16x14 Props @ 3900 motor rpm with 1.6 gear, 9000# total, resulting in 24 Knots w. 15% slip. Cruise should be at least 14 - 15 knots @ 2600 rpm ( abt. 24% slip)

(You may get by with 16x15,s but 16x14,s are ideal with 300 max HP and the heavier load)

10-4, thanks for the info!

[Jango]

With 130hp engines, props would change to 15.5 x 13p. Max top speed of 22 knots @ 3900rpm w. 9000lb total. 16% prop slip. Ideal Cruise speed could be slightly less, depending on rpm

[piperca]

Well, guys, you've been more than kind with your responses ... thanks!

Now I'm going to throw a wrench in the plan ... I've changed the engine!

I'm looking at a pair of Yanmar 4LHA-STEs (230hp each). Do you think that these engines will be too much for my boat (28'Skipjack Flybridge (http://skipjack-boats.com/history_28fb.html)) and/or my Volvo 280 outdrives? I don't want to waste my time, or the the sellers time, if they are too much for my applications ... what do you think?

[piperca]

Okay, I did some research and, in my mechanic's opinion, the drives will be fine; however, he doesn't like Yanmars ... he's a Volvo guy! Well, tough, it's not his boat!

As far as hooking up the Yanmars to the Volvo drives, this is what I came up with ... please, someone correct me if I'm wrong!

To mate the Volvo drive to the Yanmar, I'll need a flywheel cover with a Borg Warner bolt pattern and an old Volvo AQ190 or AQ240 bell housing. I'm led to believe that, since these engines were Fords, these bell housings will have the same bolt pattern as a Borg Warner gear, which will complete the puzzle from the engine to drive ... is this correct?

Is there a flywheel cover that will fit the Yanmar that has a GM bolt pattern, so that I can use the bell housings off my old AQ225Ds?

Another dilemma ... the exhaust! Is there a fitting that'll accomodate the exhaust gasses (something similar to the old Y pipe) or do I have to cut the exhaust through the transom?

Thanks in advance!

[mohamed atia]

after following all ur comments i think:

if you dont spend more money twin will be good at first installation but be a ware about running cost for your working hours.
and yanmar one will be as spend money at first and save in running cost after that.

finally i thing the two ways r the same the decision is yours only

thx for your time