Boat Yard Disagreement

[Dr Doug]

I'm new to the forum and have been researching this issues with no luck so I am here to tap into the collective wisdom of this forum.

About two years ago my Bravo one bit the dust and my boat yard guy recommended that we replace it with a Bravo 2 he had. Since the replacement the boat's performance (1989 Formula PC 26 - 7.4L with 330 HP) has not been great. Being less than knowledgable about outdrives and gear ratios, I trusted my guy when he said the issue was with the prop. We have spent the better part of two seasons changing props but still - performance is poor. While the full open thottle will reach about 4,300 rpm - I can only get the boat to reach a max speed of about 25 MPH (not KNOTS). The boat should be able to move along a a fster clip 30 to 35 mph.

I suspect that the gearing on the B2 is different than the B1 - a fact that I can confirm with Merc Cruiser. The B1 comes in 1.2, 1.5 and 1.65 gear ratios while the B2 comes in 1.8, 2.0 and 2.2 gear ratios. Last week, while I had the B2 outdrive off I tested it's ratio and it came up as 2.0 - that is 2 engine revolutions for every 1 revolution of the prop. I believe that the original B1 drive that came on the Formula was either a 1.5 or 1.65. Does anyone know what the correct gear ratio should be on an 89 Formula 26PC, single screw, 7.4 liter w/330HP or a definitive source where this information can be obtained?

My boatyard guy claims it should be 2.0 but I'm getting a little suspicious that he is incorrect. I have more than a few people suggesting that it should be 1.5.

Any help you can offer will be greatly appreciated.

Thanks,
Doug

[Jango]

If you're using the same prop, Dia and Pitch as the original set - up (1.5 or 1.65) your Engine should be Over reving with a 2.0 gear. I would guess max RPM should be 4800 +. I would make sure the gear is OK, and perhaps look at the engine - tune etc.
To determine prop size, look at the two number on the Hub. 15 x 16 etc. The second no is the pitch. Assuming your boat weighs about 5000# loaded with fuel and minimum assengers, max speed w. 330 Hp @ 4800 rpm should be abt. 38 mph.
Correct Pitch would be:

1.5 Gear - 15 P

1.65 Gear - 16 P

2.0 Gear - 20 P

(The above correlation is a coincidence - gear no. and pitch) - Doesn't work all the time

[Jango]

Unless your Boat weighs alot more than 5000#, or Gear is Bad, and/or Bottom of Boat Highly Fouled ,your Engine is developing only about 150HP. Stated RPM and speed indicates current prop is probably a 15P.
Hope this Helps,

Jango

[Jango]

After further checking, I found your Boat should weigh about 8500# with minimum fuel and passengers. This changes my previous numbers. Using a 2:1 gear which should be OK due to the weight of your setup.
Keep in mind either one of the gear choices will work with the appropriate Prop. The 2:1 is probably best and should have superior acceleration.
Max Speed will be about 30 MPH, regardless of gear.
With the 2:1 gear, recommended prop is a 19D. x 15 Pitch, 3 blade 55 D.A.R. MIN @ 4800 RPM max.( Prop would be a 16D.x 11 with a 1.5 Gear)

[Dr Doug]

Thanks for all the mesages. In my research, I have read that pitch and propeller size cannot compensate for an incorrect gear ratio. However, your note seems to indicate that the B2 may be OK, even if I currently have a 2.0 ratio when the boat may have originally been equipped with a 1.5 or 1.65 gear ratio B1 drive. By the way, the original manual says my boat is 7,000 lbs.

Anyway, I very much appreciate the information.

Thanks,
Doug

[Jango]

You are correct about prop size compensating for incorrect ratio. The slower you turn the Prop (higher ratio) the Larger it must be for a given load. BOTH DIA and PITCH. With "High" ratios this sometime becomes not practical. Often Dia is overlooked resulting in poor performance.

Also my weight calculations were based on a 26PC with Dual 120HP diesels w. a dry weight of 7750lbs - my mistake. To correctly size a prop, weight must be added for min. Fuel and passengers,say 700 lbs, making the correct total 7700 lbs.

For the 2.0 gear the correct prop is a 19x16, 1.65 gear - 17x13 and 1.5 gear - 16x12. All 3 Props will have the same slippage of 14%, which implies equal efficiency. You might not have space for a 19" dia prop required with the 2.0 gear?? Max speed with 330HP @4800rpm is 32 MPH.

[lazeyjack]

Originally Posted by Jango

You are correct about prop size compensating for incorrect ratio. The slower you turn the Prop (higher ratio) the Larger it must be for a given load. BOTH DIA and PITCH. With "High" ratios this sometime becomes not practical. Often Dia is overlooked resulting in poor performance.

Also my weight calculations were based on a 26PC with Dual 120HP diesels w. a dry weight of 7750lbs - my mistake. To correctly size a prop, weight must be added for min. Fuel and passengers,say 700 lbs, making the correct total 7700 lbs.
I always understood high ratio was 1;1, or overdriven, when you go into DEEPER ratio, 2:1, and deeper to 6:1 In other words lower ratio, the slower the screw turns,

For the 2.0 gear the correct prop is a 19x16, 1.65 gear - 17x13 and 1.5 gear - 16x12. All 3 Props will have the same slippage of 14%, which implies equal efficiency. You might not have space for a 19" dia prop required with the 2.0 gear?? Max speed with 330HP @4800rpm is 32 MPH.

I always understood high ratio was 1;1, or overdriven, when you go into DEEPER ratio, 2:1, and deeper to 6:1 In other words lower ratio, the slower the screw turns, relative to eng speed, the lower the ratio, an eng turning a gear at 6:1 is said to turn the shaft at deep reduction

[Jango]

Lazyjack, You can call it whatever you like, High, Deep, or a larger no. compared to a smaller no." i.e. 2:1, 3:1 etc. as compared to 1:1. results in a slower turning Screw.

Bottom line, - Slower turning screws need to be Larger. ALSO,Overdrive is a ratio like 1: 1.20, where the the Prop is Turning FASTER than the Engine - normally only used in Very High Speed Boats.

By the way, You are Misquoting Me - "I always understood etc" are NOT my words.