What it took for me to add a second motor

[Potomacbassin’]

My initial thought is that you have reached the potential of your particular hull/boat/motor combo.

That's where my head is as well. You can only reduce wetted surface so much by changing thrust angle before it induces proposing, and OP has hit it.

OP you're seeing additional speed as the boat is cycling to the top of the oscillation where drag is at an artificially low state. Conversely, when the boat is at the bottom of the porpoise there is a high amount of artificially induced drag and you drop in speed. So unless you can sustain that lower drag amount - less weight, more HP to get more of the hull out of the water, additional modification to achieve better planing of the tubes, etc. - hitting 53 IMO will be tough.

Maybe raising engine height or a better prop (less stern lifting design? more rake?) can get another 1-2mph? Lower the bimini?

My trick is to run with an outgoing tide with the wind to my back and 1/4 tank of gas!

 

[DVW]

That's where my head is as well. You can only reduce wetted surface so much by changing thrust angle before it induces proposing, and OP has hit it.

OP you're seeing additional speed as the boat is cycling to the top of the oscillation where drag is at an artificially low state. Conversely, when the boat is at the bottom of the porpoise there is a high amount of artificially induced drag and you drop in speed.

My boat speed does not vary in sync with each porpoising oscillation. If I over trim the first oscillation starts with vary little amplitude. The second a bit more and so on. On the first and second oscillation I can feel a steady increase in speed. My top speed is not observed until the 3nd or 4th oscillation. As the oscillations continue the amplitude steadily increases and I can feel the boat slow down. Most planing hulls will porpoise with excessive trimming but the good ones will not porpoise until after top speed is achieved. Many will aerate before they porpoise. That is the standard that I would like to achieve. I would like to hear from one of the many people who have faster boats. Could someone tell me what happens at full throttle as you trim up?

 

[Potomacbassin’]

My boat speed does not vary in sync with each porpoising oscillation. If I over trim the first oscillation starts with vary little amplitude. The second a bit more and so on. On the first and second oscillation I can feel a steady increase in speed. My top speed is not observed until the 3nd or 4th oscillation. As the oscillations continue the amplitude steadily increases and I can feel the boat slow down. Most planing hulls will porpoise with excessive trimming but the good ones will not porpoise until after top speed is achieved. Many will aerate before they porpoise. That is the standard that I would like to achieve. I would like to hear from one of the many people who have faster boats. Could someone tell me what happens at full throttle as you trim up?

My max speed of 50 was actually achieved right as the boat started bouncing along. Like you said probably around the 2-3rd bounce, but once it really starts pumping speed is lost as the pontoons dive deeper into the water and bogs down.

I've always thought that if pontoon manufacturers can create a pad of some sort in the back 1/3 of each toon that could help immensely for the higher HP boats. Or a stepped hull. But manufacturing such a design would probably be difficult - you could maybe weld on a flat sheet in the rear of the toon but of course that would also add weight. The ESP tube helps as does the lifting strakes but a flat surface would be the most efficient IMO.

Fortunately in my case a NIZPRO tune is available so if I really wanted to mess up the wife's hair it's just a phone call away.

 

[PartyBarge]

DVW, couple questions first. Have you been able to observe RPM during any intensity of the proposing? Are you still running the basic AL props. In the extreme porpoise you described, sounded like the props are oscillating between loaded and unloaded. Better props might fix that for your configuration? (meaning props that are more tolerant of aeration) There's a BIG difference between general purpose AL and high performance of any material. Good luck!

 

[DVW]

I have not been able to monitor engine speed with my focus on boat speed. I did drop the motors with no change in behavior. The props are Mercury Nemesis 4-blade aluminum 12.5” dia 19” pitch. I do have a couple of 13.5 pitch stainless props waiting in the wings. When I tried them they porpoised at the same trim angle with just a little less boat speed (48.5 mph). Engine speed was only a bit less with the stainless having more slip than the aluminum??.

 

[PartyBarge]

Ahhh...that's useful info. I have run a 23p Quicksilver Nemesis on our 190 Stingray (good for ~54 MPH with only 190 HP). It's not as good as my go-to SS props, but is a great AL compromise. Also, a 13p Nemesis is the top choice for our pontoon. In my experience, the Nemesis handles aeration well and unloads progressively without surprises. They are low-slip props with any reasonable motor height or trim.

I was concerned that you were venturing into the realm of the old twin-hull dual outboard racers. They could get dangerous and violent yaw/pitch oscillations along the lines of what you described.

If you are comfortable that your steering system and tube mounts are rock solid, it could be the only fix is a change in geometry (or go slower with less trim). With the current setup, do you have any info on what the deck angle is at speed and trimmed out just as problems begin?

 

[Bill N]

It would be so cool if a Bennington engineer were in on this discussion! It would be fun to hear his thoughts.

 

[DVW]

Ahhh...that's useful info… do you have any info on what the deck angle is at speed and trimmed out just as problems begin?

Come spring I can measure max angle of attack of the hull when it starts to porpoise. That sounds like useful information but I am not smart enough to know how to use it. I do know that when I increase the hull angle of attack the boat responds with a temporary speed increase. I think this tells me that the best lift to drag ratio is at a higher angle of attack. Unfortunately I cannot maintain that higher angle do pitch instability.

 

[PartyBarge]

Sounds like the increasing deck angle is feeding the props excessively disturbed flow. All of the really fast pontoons I've noticed here run across the water like hydroplanes...meaning low deck angle (the same seems to be the case for the fastest bass boats). However, all of those are single-motor that still achieve reduced wetted area.

It's totally speculation on my part, but addressing a flow problem at your power level may require something like more motor setback or moving them closer. The objective would be getting less turbulent flow like when using less trim. May not be worth the effort unless you are really bored and need something to do....

After running guesstimated numbers through the Merc slip calculator, it looks like 50 MPH is a reasonable limit for the 19p props. 53 requires well over 6000 RPM and a suspiciously low calculated slip. Could the 53 you saw be a transient during unstable ops?

 

[DVW]

… All of the really fast pontoons I've noticed here run across the water like hydroplanes...meaning low deck angle (the same seems to be the case for the fastest bass boats)…that still achieve reduced wetted area.

It's totally speculation on my part, but addressing a flow problem at your power level may require something like more motor setback....

After running guesstimated numbers through the Merc slip calculator, it looks like 50 MPH is a reasonable limit for the 19p props. 53 requires well over 6000 RPM and a suspiciously low calculated slip. Could the 53 you saw be a transient during unstable ops?

All of the really fast pontoons get their speed with brut force and ignorance. Seems like I cannot leave anything alone. Hydroplanes have stepped hulls and bass boats have pads. As such both boats achieve reduce wetted surface without high angle of attack. My boat does have a motor setback bracket for less trim requirement. The attached photo shows a pontoon hull with all of these elements. Unfortunately I could not use this hull do to its low displacement. As I reduced drag in steps my motor was able to maintain 6400 rpm without intervention from the rev limiter. My ‘19’ inch pitch prop does have a suspiciously low pitch. It could be the total (high) blade area but I suspect the rating is a bit skewed.

 

[PartyBarge]

Yup, the Quicksilver Nemesis (clone of Merc Spitfire) is an unusual AL high-tech prop. Your observations of low slip are similar to what I've seen with both of my Nemesis props even though they are for different drive case size. And yes compared to some other props, the performance is out of the ordinary in at least some applications. I expect to never rebuild one (even if cost were not a factor) because I doubt a rebuild would match the factory contours.

Increased load causes speed loss, but the way that happens can be different. I've noticed that increased load reduces engine RPM for my Nemesis props, but has little effect on slip. That is not the situation with many props, including some of my SS props. For many increased load increases slip with less RPM change.

 

[mtudb24]

All of the really fast pontoons get their speed with brut force and ignorance. Seems like I cannot leave anything alone. Hydroplanes have stepped hulls and bass boats have pads. As such both boats achieve reduce wetted surface without high angle of attack. My boat does have a motor setback bracket for less trim requirement. The attached photo shows a pontoon hull with all of these elements. Unfortunately I could not use this hull do to its low displacement. As I reduced drag in steps my motor was able to maintain 6400 rpm without intervention from the rev limiter. My ‘19’ inch pitch prop does have a suspiciously low pitch. It could be the total (high) blade area but I suspect the rating is a bit skewed.
View attachment 33759

Maybe I missed it, but what is that hole on the inboard side of the port side pontoon to the right of the motor mounting plate? Do you have that hole on the starboard tube as well? Looks like you are also running 2 upper bolts and 1 lower bolt in the engine mounting plate? Just curious.

 

[DVW]

Maybe I missed it, but what is that hole on the inboard side of the port side pontoon to the right of the motor mounting plate? Do you have that hole on the starboard tube as well? Looks like you are also running 2 upper bolts and 1 lower bolt in the engine mounting plate? Just curious.

The photo that you are referring to is not the hull on my boat. It is of a manufactured tube with elements of a stepped hull, a pad hull and my offset transom. Bringing these elements together could eliminate porpoising. Unfortunately this particular tube lacks displacement and planing surface for my two tube boat. I chose to modify Bennington elliptical tubes (see photo).

 

[Bill N]

I'm convinced that the only way you can go faster is to have less boat in the water, and the only way to get less boat in the water at this point is to go faster.
So basically, you're gonna have to go faster in order to go faster!

 

[DVW]

I'm convinced that the only way you can go faster is to have less boat in the water, and the only way to get less boat in the water at this point is to go faster.
So basically, you're gonna have to go faster in order to go faster!

Love that circular thought.

I am thinking I need to get less boat in the water by increasing the angle of attack in the water (trimming up). That will reduce the drag caused by surface friction (parasitic drag). Unfortunately the increased angle that creates more lift comes with more drag induced by the lift (induced drag). That is OK as long as there is a worthwhile tradeoff. Most well sorted out planning hulls can be trimmed up to the point of diminishing returns. My boat is not well sorted out yet. It starts to porpoise well before that point.

 

[Bill N]

If reducing the wetted area results in the remaining wetted area plowing deeper in the water it could actually result in a loss?

 

[DVW]

If reducing the wetted area results in the remaining wetted area plowing deeper in the water it could actually result in a loss?

Yes, good summary.

 

[PartyBarge]

If reducing the wetted area results in the remaining wetted area plowing deeper in the water it could actually result in a loss?

+1 The objective is to move as little water as possible. Excessive deck angle (aka pontoon angle) is redirecting too much water AND feeding the props too much turbulence.

 

[DVW]

+1 The objective is to move as little water as possible. Excessive deck angle (aka pontoon angle) is redirecting too much water AND feeding the props too much turbulence.

The objective of a displacement hull is to move as little water as possible. The objective of a planing hull is to reduce drag by displacing water downward in order to generate lift. There are tradeoffs associated with either hull. My goal is to get as much speed as possible with modest power. At this time I cannot trim up much without porpoising. When I fix that if my slip numbers go up I will drop the props a notch.

 

[PartyBarge]

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.