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

[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]

Originally Posted by Jango

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]

Originally Posted by 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.

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.

[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]

Originally Posted by Karl2

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