Perfomance issue

[kck]

Recently purchased a new South Bay triple tube Pontoon with lifting strakes, 26'4" long 8'6" beam. Volvo Penta 5.7 GXI-H 320hp, SX-A stern drive. Boat weighs approx. 5500lbs fully loaded (fuel, water, gear, people). Prior to purchase the boat manufacturer and the dealer stated a cruise speed of 30-32mph and low to mid 40's at wot. Originally sea tested with a 1.43:1 ratio, 15 x 15 prop and achieved 4500rpm/38mph at wot. Dealer stated re-propping was necessary to achieve stated speeds. At time of re-propping dealer claims at the time of manufacturing the wrong outdrive was paired with the engine. Dealer installs a new SX-A sterndrive with a 1.60 ratio and 15 x 17 prop. With a full tank of fuel and 3 people on board Tach and GPS readings showed 3000rpm/22.5mph, 3500rpm/26.5mph, 4000rpm/30.5mph, wot 4200rpm/32mph. Boat was returned to dealer for 20 hour service and re-propped with 15 x 15 prop. With full fuel and 1 person on board Tach and GPS now read 3000rpm/21.7mph, 3500rpm/25.5mph, 4000rpm/30.2mph, 4500rpm/35.4mph, wot 4800rpm/37.5mph (4 people on board 4800rpm/36mph). I'm a beginner at this. Any comments or suggestions would be greatly appreciated.

[Jimboat]

with 1.6:1 gears and 4800RPM, 9% slip estimated would give you 38.8mph actual speed. With 4 people in boat, this seems about right. With lesser payload, you may be able to achieve higher RPM, and thus better speed. Your engine has a published Max. RPM of 5000RPM Rating, so you still have room for larger propeller (more pitch) if engine can swing it...but if your max. RPM that you can get from the engine is still less than Rating even with the lower payload, then you'll need to try a different propeller design, or smaller diameter (eg: 14"diamX16"pitch). Seems like you are close with your current setup...might get 40mph with higher RPM and prop change.

[kck]

Jimboat- Thanks for your comments. I communicated with Vovo and they suggest a 1.43 or 1.51 gear ratio with a 14.5 x 21 3 blade alum prop for our Michigan altitude.

[Jimboat]

At 1.51:1 gear ratio and , if you were to be able to run at Max RPM Rating of 5000 RPM with the 21in prop, then you would see 59.9 mph (at 9% slip). The 14.5in diam prop still seems pretty big to me, so it would be quite good if you could actually turn 5000RPM with a 21in pitch prop, based on the performance information that you've provided so far. Nevertheless, a smaller diam. and larger pitch 3-blade will help you. when you're testing propellers, be sure to get your supplier to allow you to test several props, and try them all, recording all your results. I expect you will see best results with smaller diameter (14 in) and slightly larger pitch, and speeds up to 45mph. Call if you need any help.

[kck]

Thanks Jimboat. What performance might I expect with a 1.43:1 gear ratio? What prop would you recomend with this set up?

[Jimboat]

kck - I find it hard to see that a 1.43:1 gear ratio would work, based on the RPM and performance information that you have provided so far. If, however, you were actually able to turn the 21in prop at a full 5000RPM with your 1.43 gearset, then you would get 63.3 (assuming 9% slip). Since you say you are only able to turn 4800RPM with a 15in/15in prop, I wonder how you could turn 5000RPM with a 21in prop that is only a little smaller (14.5in diam.)

[kck]

Jimboat-I would agree with your assessment. I was getting conflicting info from Volvo which didn't make a lot of sense to me and wanted another opinion. I'm going to follow your suggestion and try a 14 x 17 & 14 x 19 prop with my current 1.60:1 gear ratio. I'm hoping it will help increase my cruise speed and economy. Thanks for all your comments.

[Jango]

First, Prop should be sized at max.rated RPM with One person on board


According to "PropCalc"

Displacement 5000lb (one person), 320 HP, w. 1.43 gear (higher gear w. slower shaft speed will require larger Prop)

Prop size should be 15 x 13, 3 blade 63% DAR min.

Max Speed will be 38.4 mph @ 5000rpm and 11% slip (Best Posible)

Hope this helps - Jango

[kck]

Jango-Thanks. Sorry, I must have confused the situation. I actually have a gear ratio of 1.60:1.

[Jango]

With a 1.60 gear, your no,s are as follows.


Prop: 16x15 w. 55%DAR min. 12% slip Max. speed @ 5000rpm

Notice with higher gear, (slower shaft),slip, has increased slightly - lower effeciency

Max. speed : 38.4 mph (unchanged) w. 320HP , 5000LB.

I believe 14 Dia props will cause excessive slip with this gear w. far less effeciency.

[kck]

Jango-Thanks for your comments.

[Jango]

You're welcome KCK.

If the HP and weight no's provided are correct, I am very confident in the calculations obtained in my last two posts.

Jango