Pontoon power question

I just met a guy at Lake Tahoe that has a U-Fab pontoon boat. It’s 32’ long and 12’ wide with 2- 36” diameter round pontoons. We started to talk about how fast it wasn’t.
When he bought it (used) it had a 115 hp and he said the boat would max out at like 8-9 mph. A real slug. So he and his partner had the engine mount beefed up and bought a new 300hp outboard.
He says it now maxes out at about 12-13 mph. Wow!
They are now talking about having the center engine mount removed and adding mounts to the back of rack pontoon and adding a second 300hp outboard.
I asked if the boat had lifting strakes and he said it only had little splash rails near the bow.
Wouldn’t he be better off having full length lifting strakes to the hull before doubling his HP. Especially given the spectacular failure of the first motor upgrade?
I now realized I should have asked him about how submerged the hulls are at rest to get some idea of how heavy the boat is. It does not have and cabin on it but it has an upper deck on the rear half and I’m sure the typical pontoon boat furniture.
He mentioned he would like to see the boat do 20-25mph.
Would strakes get him there?

 

Yes lifting strakes should get him to 25 mph unless something else is going on

My old 24 ft pontoon with no lifting straked did 25 mph with a 115 hp.

 

Hi.

The Conrad Marine TAP Fin System promises to be more efficient than strakes.
How It Works https://www.conradmarine.com/how-it-works/

 

Interesting. I wonder why this isn’t the standard now.

 

U-Fab told me that a 36’ x 12’ pontoon boat with 2- 30” pontoons should be able to do 30 knots with 2- 50hp outboards.
This is with a 2700 lb shipped hull weight and a total weight of 10,500 lbs (hull, power, equipment, passengers).

 

There is no way it would do 30 knots with (2) 50 hp outboards.

 

That’s what I thought. And now I've met someone with almost the same boat, (36” pontoons vs 30” I was quoted) and he can’t break 15 mph with a 300.
My quote did include full length lifting strakes on both sides of both pontoons, but still, something isn’t right.

 

How to calculate power for pontoon boat. This is from the Canadian Standard, which is the same as the ABYC standard for pontoon boats.

4.5.5 The Recommended Maximum Power Calculation

4.5.5.1 The recommended maximum power in kilowatts marked on a capacity label for an outboard powered pontoon vessel is determined in relation to the length of the pontoons squared and the diameter of the pontoons as follows:

MaximumPower kW = 3 X Lh X Lh X Dp

Where:

Lh = vessel length of the pontoon in metres as per Figure 0-1

Dp = diameter of the pontoon in metres

4.5.5.2 When the calculated power is not more than 10 kW (15 hp), the power may be rounded up to the next multiple of 1.5 kW (2 hp). When the calculated power is more than 10 kW (15 hp) the power may be rounded up to the next multiple 3.75 kW (5 hp)

Convert Kilowatts to HP
P(hp) = P(kW) / 0.745699872

 

So using this formula for the 32’ x 36” pontoon in question, I come out with approx. 195 hp. Let’s call it 200 hp.
So now I’m wondering if this calc is per pontoon or if this is for pontoons in displacement mode, because it doesn’t seem like much HP. Especially when you see how many new pontoons you can buy with 300-400 hp outboards.

 

I tried to find this standard on-line and I believe what Ike referenced applies to pontoons below 20’. Above 20’ it states “shall be determined by the manufacture”.
So my calc above doesn’t seem to apply.

 

Google search Crouch formula. It will help predict the HP needed for the speed you are looking for.

Check my math, lol. But given 2 x 200hp and 10,500 lbs displacement the Crouch formula shows a 29 kn top speed which is probably a good 20 kn cruising speed.

The constant I used is 150. It is what I have noticed to be true for pontoon boats with lifting strakes.

 

It's easy to convince people that something simple and low cost can be as or more efficient than the traditional one. Just show them.
The problem is to convince them to come down from their pedestals as connoisseurs so that, in their arrogance, they can be at the
height of any mortal. These, by the way, are the ones that kill good ideas still in their nests.