Advice on a PAD

[chris3298]

I talked to Glenn Hopkins and his advice for me either to try extending my trim tabs across the length of the transom and make them adjustable or welding a pad to the bottom of the boat. He said make it 5 ft in length and about 14 inches in width. I'm curious to know could I make it 10.5 inches in width? or even 12 inches possibly?
10.5 would be better for the way the hull comes to a V at the stern and the chimes are so dang close together.
I would think the narrower the pad the faster it would go but harder to control correct?
would my 50 tohatsu have a hard time getting the boat onto a 10.5 inch pad??

What advice can yall give me? I hadn't talked to Glenn in a couple of days and been brain storming this.

Thanks
Chris

[bad news racing]

Chris , the smaller the pad the more horse powe it takes to get it up and keep it there

[Nason]

Chris , the smaller the pad the more horse powe it takes to get it up and keep it there

I agree with the horse power to a certain degree on keeping it on the pad. my boat has a 14" pad with a 40 hp. With a regular prop without trim you would have to ride it through the bouncing until it was on the pad, added a hydraulic jack plate came up on engine 2.5 inches
rides good from 35 mph to 44 mph any slower it wants to fall off pad. With chopper prop at 40 mph it want to fall off pad at 52 its on pad and holding.I think I could come up more on the motor but need to get used of driving this boat first.

[chris3298]

Chris , the smaller the pad the more horse powe it takes to get it up and keep it there


what is your advice then go with 14 inches?

[Ron Hill]

Bottom Line: We have 15 HP motors with 36-39" pads running 50 MPH. Old Hydrostreams ran a 4 inch pad with 150 HP. But with a 40 HP, 30 inch pad would be better than a 14" pad. 10 inch is a "NO NO"...

Boats have four stages: 1. Float 2. Plow 3. Plane 4. Fly For speed, you want to fly!

[chris3298]

Bottom Line: We have 15 HP motors with 36-39" pads running 50 MPH. Old Hydrostreams ran a 4 inch pad with 150 HP. But with a 40 HP, 30 inch pad would be better than a 14" pad. 10 inch is a "NO NO"...

Boats have four stages: 1. Float 2. Plow 3. Plane 4. Fly For speed, you want to fly!


Thanks Ron, it's better to ask all the questions now before i get down the road and have a problem. Just gonna have to do more figuring this weekend and may just have to make a 14 inch work or even a spec more

[chris3298]

Haven't done the pad yet but I have all my material and found a guy couple days ago with a brake to bend my sheet into a channel and well I was talking to someone last night I think his boat is 15 ft in length and he said his pad is about 12 ft in length under the boat.
Do you think i'm gonna have a problem with this pad being just 5ft in length or should it be longer? I hope not because I have everything setup and ready to do it but am having some second thoughts here on the length of the pad going forward. My pad is currently 5 ft in length.

Just like a little more advice would this length be ok?

Thanks
Chris

[Jimboat]

I'm curious to know could I make it 10.5 inches in width? or even 12 inches possibly? I would think the narrower the pad the faster it would go but harder to control correct? would my 50 tohatsu have a hard time getting the boat onto a 10.5 inch pad?
The size of the pad should be designed so that it's sufficient to provide greater portion of dynamic lift of the hull. A wider pad is more efficient, but also harder ride. Hulls with high deadrise vee will benefit more from having a pad, low deadrise hulls will not see the same benefits. For any hull/setup, pad dimensions can be optimized for the hull. Optimum width and length are sized based on hull weight, power and dynamic balance.

Here is Article on Vee-pad design (http://www.aeromarineresearch.com/publications/RIB_jun2011.html) .

[chris3298]

The size of the pad should be sufficient to provide greater portion of dynamic lift of the hull. A wider pad is more efficient, but also harder ride. Optimum width and length are sized based on hull weight, power and dynamic balance.

Article on Vee-pad design (http://www.aeromarineresearch.com/publications/RIB_jun2011.html)

Thanks for the article

[Jimboat]

no problem, chris. give me a call or PM me, if you want to discuss.

/jim

[Smokin' Joe]

Not necessarily. If you stop and think you'll realize there must be a limit,
because a pad V is faster than a flat bottom, all other things being equal.
On the 13' Allison with OMC 75 an 8" pad was too wide, the boat ran deeper at the
transom. With a 7" pad the boat gained speed and turning ability. The original pad width was
5.5". The problem with an 8" pad is that it was too deep, that caused the extra drag that allowed
the boat to sink at the transom. The pad was too deep because the Allison is built with just
the right degree of deadrise to allow the boat to bank and turn well. Look at an old 14' Allison
'with a 7" pad. You don't want the pad to be any deeper than that. Otherwise, you're dragging
extra water.

[Smokin' Joe]

My 14.5' Allison weighs about 350 lb and runs about 42 mph with a 1983 Johnson 35,
after a lot of triclky prop work. the pad width is 8". A wider pad would not be faster,
the boat already runs too wet because 35 hp is not enough to get it on the pad. I
would not widen the pad, I'd add lift strakes off the pad designed to make the flow more two
dimensional. that would lift the boat at the pad.

I think that there is no rule from hydrodynamics that tells us what is an optimal pad
width. Were there such a rule then I would have been able to derive it mathematically.

The Professor

[Smokin' Joe]

I'd hate to think of trying to straightedge a steel or val pad. Putting a pad
on a flat bottom won't work, and it won't be of much use on a semi-V with small deadrise.

[Jimboat]

Every hull design/setup has an optimum pad design (width, length, depth) for the desired performance expectations. The tricky part of the design is that the characteristics change at each velocity throughout the full operating velocity range of the hull.

So the design of the pad will always be a 'compromise' that performs "best" at one speed and condition. Never-the-less, the performance of any pad design can be predicted by analysis.

[Smokin' Joe]

Every hull design/setup has an optimum pad design (width, length, depth) for the desired performance expectations. The tricky part of the design is that the characteristics change at each velocity throughout the full operating velocity range of the hull. So the design of the pad will always be a 'compromise' that performs "best" at one speed. Never-the-less, the performance of any pad design can be predicted by analysis.

Hi Jim, I could agree with you if there were a way to calculate that, but the problem is highly
non-unique, is ill-posed. E,g., what pad width and depth would I need to run an 840 lb rig 65 mph with a
Johnson 75? There are a lot of different answers to that question, in practice. If I add the constraint of
wanting to turn and run rough water well then I may narrow the field (as I know from experience, but
not from calculation), but there is still no single answer. Best, Joe

[Jimboat]

E,g., what pad width and depth would I need to run an 840 lb rig 65 mph with a
Johnson 75? If I add the constraint of wanting to turn and run rough water well then I may narrow the field, but there is still no single answer.
Joe - true enough. As i suggested, nearly every hull design is a "compromise" of many performance desires/expectations. Given an existing vee hull design of weight/size/power - the best pad design is a compromise of the design for ultimate speed and the design for handling/turning. Acceleration could be a consideration too, which also would be part of the 'compromise'. Each of these concepts have a pad design that can be calculated and performance predicted. The designers job is to select the "compromise" design that best "balances" the needs for ultimate speed and handling/turning that will meet the overall intended uses of the hull.

As you will know, the vee-pad design that does well overall on a closed course (with turning important) will likely be different than the vee-pad design that does well on a straight-line kilo course intended to set top speed mark.

The result can be complicated further if there is a need for 'comfortable planing' at intermediate speeds; or stable speed control at, say, waterskiing speeds.

It's all very 'calculatable', though. All the parameters can be considered, and there's a method to calculate all the loads, lifts, drags, stability and performance. But in the end it's still a compromise - and the compromise is a single design.

Luckily, it can all be fun to do!