What it took for me to add a second motor

[DVW]

Actually moving as little water as possible is still a prime objective even for our 55+MPH Stingray. One of the reasons it makes that speed on only 190HP is that the I/O power package is mounted relatively high reducing the amount of the lower unit in the water as compared to most I/Os. To be fair, another reason it has speed is there is only ~14 degrees of deadrise at the transom. Too much trim at modest speeds causes porpoise, but at high speed high trim does not change the deck angle much but does get more of the lower out of the water (up until surface turbulence causes the prop to unload). Some props unload in a controllable progressive way, others are abrupt. Another way to look at it is that trim simply better aligns the trust vector with the direction of movement considering the natural deck angle of the hull at speed (which is relatively flat). Generally, using thrust to achieve a deck angle is not the best use of power for modestly powered boats.

To be sure, lighter outboards can beat my speed on similar power, but I/O's have their advantages too.

Thank you for reporting the data on your Stingray boat. Its response to trimming at two different speeds is most interesting. More to think about.

 

[rbtnt]

Do you still have the tabs intalled that you show in post #20?

 

[DVW]

Do you still have the tabs intalled that you show in post #20?

The trim tabs where removed after I determined that they did not reduce the porpoising.

On another related topic, on a earlier post I reported that I have stainless props waiting in the wings once the porpoising problem is sorted out. The reported pitch of these props was in error. These 21” pitch stainless props where purchased to replace the existing 19” aluminum props once the purposing problem is resolved. I still do not have a clue how to fix porpoising. What I am looking for is a boat that when the motor is trimmed up (at full throttle) it will reach its top speed before it porpoises. Many well sorted out fast boats will do this but many do not. I will keep trying to fix this on my boat. Any help would be appreciated.

 

[rbtnt]

Thanks for the update. I have 3 friends with high HP single enigine tritoons who are working on a similar problem when they reach mid to upper 50s. One of them is having luck with a sport master gearcase. These boats are not Benningtons, but have the same issues you have with the porpoising.

 

[PartyBarge]

The info in the link below may be useful:

The Right Gearcase: Sport Master, Torque Master, Nose Cones — Wave To Wave

Your gearcase is critical to your boats performance. With so many gearcases out there and some cone options, sometimes it’s hard to figure what is best

www.wavetowave.com

This may be worth a look too:

Hydrodynamics: Part 2

HYDRODYNAMICS: PART 2 - THE SCIENCE AND ART OF GEARCASE HYDRODYNAMICS In our recent Intro to Hydrodynamics post we discussed the impact of drag on escalating boat speed, and the importance of using the science of hydrodynamics to reduce that drag. The gearcase is the one component of a...

community.brunswick.com