plavutka

And I would never put a bb mastodon in my speedboat. (I do not looking for 80+ Mph) This about the torques, the size of the boat and the inadequacy of the LS motor is often taken out of context on this forum. (Outboards with 400 or 500 hp do not have better torque than LS engines, all of rhem are surfacing "engines"...)

1.) I believe LS engines don't work well with Z drives because all decent Z drives have the biggest gear ratio of 1.6:1, which is too low for an LS that revs to 6,400 rpm and put out the best power at 5,800 to 6,200 rpm. In this case, you get about 4,000 rpm on the propeller, which means very little torque and this is only suitable for very light and fast speedboats, with less than 3.500 lbs weight for each engine.
2.) If you put a 2:1 gear between the LS engine and the propeller, you get, in my case with the LS376/480, torques on the propeller that are very comparable to the torques of the Merc. 525 EFI and 1.5:1 gear drive.
3.) Check out the Volvo Penta inboard motors for example!
4.) LS engine and 2:1 transmision give similary prop rpm like BB with 1.6:1 transmision.

Our speedboat pulled like a snot with the 1.56:1 gear. It took 7 seconds to come out of the water and 12 seconds to 20 Mph, but when we put in the 2:1 gearbox and the Maximus 30, it reaches 20 Mph in about 6 seconds and up to 65 Mph the first results are very comparable to the Velocity SC29 test with 8.1 HO, which seems ok to me for the first tests. I guess we will need a 28" propeller in the end.

A very simple calc that is good enough for comparison is:
P = D**** x n*** x H** x A
P = needed power to turn prop. In this case only like factor to compare diferent situations.
D = propeller diameter
n = propeller rpm
H = propeller pitch
A = propeller area
told me that with a Signature LS 22 propeller and a 1.56 gear, I put out 485 hp, and the same work was done that would have been done at the same speed achieved with a 2:1 gear, a 30" propeller, and 375 hp!
Due to the simplification of the formula, you do not get an exact power result, but it is enough to compare different situations.

Not to go too far, I would like to share my next opinion:
If you have the possibility of using a gear ratio of at least 1.8:1, even better 2:1, the LS engine is at least as good as the BB with much lower consumption. With Z drives you must have less than 3.500 lbs boat weight per engine.


p.s.
I hope the text is understandable, I didn't mix up the words too much and at the same time I apologize for my English.

Rookie

Throw a lot of extra money at it and hope it performs.

plavutka

The alpha drive has a 2:1 gear ratio.

Rookie

What Alpha drive are you referring to? He never listed a serial #.
The have:
1.47
1.62
1.81
1.94
2.00
2.40

plavutka

With 2:1 you will have better torque on the propeller than 8.2 HO with 1.6:1 and maximum propeller revolutions around 3,000 rpm.
With 1.94:1 you will have identical conditions on the propeller as with 8.2 HO and gear 1.6:1.
With 1.8:1 you will have similar conditions as with 8.2 HO and gear 1.5:1.
In all cases will be consumption of LS smaller becouse of better engine construction than BB and SB.
I do not know what will be the best for you, but I would looking betwen 1.94 ansd 2:1.

Depending on the weight of your vessel, search between these gears. Less mass comes on one motor with a smaller gear. I don't know how much the Bravo drive can handle, and with 1.8:1 it holds more than with 2:1.
About how much would your speedboat weigh? Check this: https://i.postimg.cc/jjTkKhHP/47713064-3.png it is just marine dresed LS3 or LT engine coupeled on Z drive.

You have to put aluminum exhausts on the engines because of the electrolysis (We put Kodiak.), which is the biggest additional cost. It will definitely be much lighter than SB.

Like I told: We have 4950 lbs heavy boat 30# LOA and id ride well with 2:1, MSA surface drive (Ugly but good except skeg which we need to prolongate.) http://www.tuprop.com/ and at the moment Maximus 30, probably will be at the end 28, but I do not know yet.

SB

You guys know how much power and load a 2:1 Alpha dive will handle ?
This is what the OP has

tommymonza

Back in the day I replaced a broken 1.47 that was behind my stock 260 with a 1.81 from a 4 cylinder that was laying around. Increased the pitch to get some of the speed back. The outdrive lasted a month before it broke the vertical shaft but that was the old MRs.