Hull form versus engine power and torque In maintaining a boat on plain

[Jimwhall]

So most of my life experience has been at the 16 foot sylvan aluminum hall. It is 112 hp Johnson. It's been a great boat for me, but now that I have been on some more fiberglass boats it seems like the glass boats with stern drives do a better job planing at low speed. This puzzles me because the hulls are much heavier than the one in my Sylvan.

Is this because the stern drive engines generally have more torque than my two-stroke outboard, and so can push a bigger prop? Or does it have more to do with the fact that they are significantly beamier and have more complex hull forms?

This isn't a huge deal obviously, I just want to better understand.

 

[JimS123]

My experience is just the opposite.

 

[southkogs]

I'm going to go with hull form (for the most part). The right power on the right hull to get the right performance.

I would have agreed with Jim mostly, except that my current Bayliner is actually really good at getting up out of the water quickly and planes at relatively low speed.

 

[Chris1956]

Planning at low speed is a function of the hull design and available HP at a given RPM. So a boat with a lower pitch prop, will have more power available at a given RPM, and will plane at a lower speed than the same boat with a higher pitch prop, provided the hull will allow the lower planning speed.

Planning hull Boats have a displacement speed and a minimum planning speed. They cannot run in between those speeds easily. With constant throttle adjustments, they can run at some in between speeds, but not all.

In conclusion, the type of power doesn't directly affect the speed of planning, however, it indirectly affects it.

Sterndrive boats are generally more stern heavy and would need more power to plane. Power can be related to displacement and sterndrives generally have more of that per given power rating than OBs.

So if you had two identical boats, one with OB power and one with I/O power of the same HP, and you weighted them the same, bow to stern. The sterndrive would likely have a higher displacement, giving it a bit better low end power and would likely plane at a lower speed.

If you didn't weight them the same bow to stern, the sterndrive would likely drag it stern more than the OB and need to plane at a higher speed.

 

[ahicks]

I would need to add a bit to this: "Planning at low speed is a function of the hull design and available HP at a given RPM."

To be added would be WEIGHT and further, prop design. A 4 bladed prop for instance, provides more lift, increasing a boat's ability to stay on plane at lower speeds.

Regarding weight, it should be easily seen 2 boats with identical power and hull designs, one weighing 500 lbs less than the other, will have the lighter boat holding a planning speed longer than the heavier boat.

 

[JimS123]

I would need to add a bit to this: "Planning at low speed is a function of the hull design and available HP at a given RPM."

To be added would be WEIGHT and further, prop design. A 4 bladed prop for instance, provides more lift, increasing a boat's ability to stay on plane at lower speeds.

Regarding weight, it should be easily seen 2 boats with identical power and hull designs, one weighing 500 lbs less than the other, will have the lighter boat holding a planning speed longer than the heavier boat.

Your "lighter boat" theory would seem to blow the OP's theory out of the water.

Your 4-blade prop comment is inconsequential because that's an after-market product designed for a specific need. When did you ever see a factory boat with an OEM 4-blader?

Fiberglass boats today are built and sold for SPEED. If you can't go faster than your dock mate, you won't buy the boat. (stupid reasoning but nevertheless all too common).

Many of the designs also require more HP. It's a catch-22 kinda thing.

Back in the day, we all had woodies and were perfectly happy with a 25 HP OB. They could plane and ski all day long. My buddy's Dad had an 18 footer with a 75 HP OB. OMG, we thought we were in heaven.

Wooden boats were well designed as well. With the lighter weight, the buoyancy of wood and awesome hull designs nobody ever talked about planing speed.

A good corollary is a Piper Cub vs. a Jet. The piper will fly by itself even with the engine off. A jet requires full thrust just to keep the sucker in the air.

 

[Stinnett21]

A 16' with a V4 probably two batteries in the back and gas tank positioned toward the back and with only about 14 feet of planing surface in the water is a difficult set up to keep on plane at slow speeds. Trim tabs would help greatly. But then you run into interference with boarding ladders, transducers, etc.

 

[JimS123]

A 16' with a V4 probably two batteries in the back and gas tank positioned toward the back and with only about 14 feet of planing surface in the water is a difficult set up to keep on plane at slow speeds. Trim tabs would help greatly. But then you run into interference with boarding ladders, transducers, etc.

I interpreted the question as more theoretical that needing an answer on how to improve.

Certainly, trim tabs are a potential fix, but adding a DoelFin is truly the optimum solution, coupled with possibly engine height optimization.

No holes to drill in the boat, no interference with other equipment and a cost no more than pocket change.

 

[ahicks]

Your "lighter boat" theory would seem to blow the OP's theory out of the water.

Your 4-blade prop comment is inconsequential because that's an after-market product designed for a specific need. When did you ever see a factory boat with an OEM 4-blader?

Fiberglass boats today are built and sold for SPEED. If you can't go faster than your dock mate, you won't buy the boat. (stupid reasoning but nevertheless all too common).

Many of the designs also require more HP. It's a catch-22 kinda thing.

Back in the day, we all had woodies and were perfectly happy with a 25 HP OB. They could plane and ski all day long. My buddy's Dad had an 18 footer with a 75 HP OB. OMG, we thought we were in heaven.

Wooden boats were well designed as well. With the lighter weight, the buoyancy of wood and awesome hull designs nobody ever talked about planing speed.

A good corollary is a Piper Cub vs. a Jet. The piper will fly by itself even with the engine off. A jet requires full thrust just to keep the sucker in the air.

Maybe I read incorrectly. I thought we were discussing the ability to stay on plane at the lowest speed?

 

[dingbat]

Plain and simple….hull design determines planing speed. A hull gives x amount of lift for a given planing surface. If the boat is designed to plane at 10 kts. It doesn’t matter how you get to 10 kts. Only that you get there.

To change… increase planing surface or decrease weigh per given hull area.

 

[Jimwhall]

Wow, what a bunch of great answers.

I was out for a while but it's really been a pleasure reading these. To just a couple points brought up I wasn't viewing this is a problem. I did just wanted to know more. However, the points made about the Sylvan carrying all of its weight in the rear really did go a long way to explain things. It seemed to get really affected by towing plus having the gas tank and engine and batteries in the back. I have to have a very deft touch on the throttle if I'm telling someone who doesn't want to go that fast.

I did get some trim tabs, and I got a four bladed prop with a lesser pitch and that did help some. I'm now just more nervous if other people drive the boat because it's fairly easy to over rev the engine in the set up.

But these answers did explain a lot. Hydrodynamics is always fascinated me. And what they do with hulls nowadays is amazing. I do remember having a small wooden boat with a 35 hp outboard and that really seemed to do pretty much everything we wanted. It wasn't overly fast, but it could haul up my brother and sister on skis when they were kids. I do remember seeing a 75 horse outboard back in those days and thinking it was an absolute monster. Of course our "big boat" back in those days was a 14 foot Larson lap liner LOL in the wooden boat had an alternate pulse start as well as could run almost a whole weekend on a gas can that my dad carried into the boat.

 

[dingbat]

I did get some trim tabs, and I got a four bladed prop with a lesser pitch and that did help some. I'm now just more nervous if other people drive the boat because it's fairly easy to over rev the engine in the set up.

Running an under-sized prop is counter productive. Your leaving much needed Horsepower on the table by limiting the output of your motor through prop selection.

 

[Jimwhall]

Running an under-sized prop is counter productive. Your leaving much needed Horsepower on the table by limiting the output of your motor through prop selection

By reduced pitch? I ran one pitch up (15 to 17,I couldn't find a 16) and it really lugged while townig the kids.

 

[ahicks]

By reduced pitch? I ran one pitch up (15 to 17,I couldn't find a 16) and it really lugged while townig the kids.

Dingbat seems to be fixating on top speed.
When setting up for top speed, you most often leave towing performance on the table.....

It's a case of 'druthers! When set up for one, you loose on the other!

The ONLY caution when set up for towing, is with most boats, you need to keep an eye on your tach to prevent over revving when lightly loaded/not towing. -Al

 

[Jimwhall]

That is something I'm very careful of. I actually was going to go higher in pitch to make sure that my engine would never over rev. But I could never live with the low-end performance cost.

 

[Stinnett21]

Good idea to have more than one prop. Go with low pitch on heavy load water sports days and switch to higher pitch on lighter load cruising days. When you have all the tools at hand to remove/install can be done in 5 minutes.

 

[dingbat]

I actually was going to go higher in pitch to make sure that my engine would never over rev. But I could never live with the low-end performance cost.

Did you look at ported props? They where designed specifically to address your situation.

 

[Jimwhall]

Did you look at ported props? They where designed specifically to address your situation.

The turning point I have is ported

 

[Beagleville]

I think hull design is one of the biggest factors in ease of planing. A flat-bottomed boat will plane much more easily than a deep V. You can skip a flat rock across the water easily, but not a round one--unless you could somehow launch it at three times your arm strength. The trade-off with a flat versus a V is ride quality--flat will pound in a chop whereas a V will give a much better ride but use a lot more horsepower in doing it. The old wooden boats planed easily but didn't ride nearly as well as a modern deep V, but back then a 75 hp was huge, today it's nothing at all.