top speed estimator
 

Ok, someone posted a web site that enabled some calculations to estimate top speed, using a hull factor, HP, and weight. It also had the slip calculations. I was sure I bookmarked it, but cant find it. Anyone know of this??

thats it
 

Thanks, and now while re-saving it and found out that I did have it. It shows up as "mercruiser parts at BAM marine" which did not sound familiar. I guess I have too many sites saved for my own good. Its not what you know, its knowing how to find the answer, and I failed on both accounts!!

So, you got something planned for your boat? A little horsepower upgrade maybe? Or you thinking about that 34' Mach1 still?

Shorty

no changes to the current one. I did test drive an Envision yesterday. 32' with single 496HO. I wanted to see how the salesman's claim compared with the speed calculation. I found pretty good coorelation with that web page's formula and my boat.

Yep, when I add in the weight of boat + people + gas + a 100 lbs of beer & junk, I find it to match my GPS exactly. Unfortunately, I have to use the slow v-hull constant. :(

But if I had to guess, that 32' envision with a single 496 will be lucky to hit 50mph with 4 people and a half tank of gas.

who is going to carry the Mach1 now? anyone at the lake?

That Envision sure rode nicely yesterday, hardly ever felt the rough water. :D

But, if everyday was like yesterday I would be looking at a ski boat instead ;)



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with the single, its lighter than the typical 32 (6500# ish) so using 7300, 425HP the calc says about 54mph. Dealer said about mid 50s, fairly honest guy I guess. I usually think of madatory twins at 30" or so, thats why I am especailly glad I drove it.

So it must have the 496HO not the regular 496. A whippled 500efi would probably move that along real nice. :)

I don't know who's going to carry the Mach's down there now. I have seen some used 34's with twins for sale though. But, then my opinion is slightly biased ;) You could order direclty from the Baha Cruisers factory in Florida I suppose. Might even be cheaper.

Shorty

Only problem with that gizmo is that it leaves out certain critical variables, to wit:

1. Place where the analysis is taking place, and
2. Number of liquid refreshments consumed

It is a well known fact that any boat is faster at the dock than out on the water, and that this speed differential increases with the number of boat sodas consumed.

A similar effect is seen with water conditions. This explains how an 18' boat with a 4.3 v-6 can run 75 mph in 10 foot seas when you're sitting behind 4 empty bottles at the bar ;)

At that point the BS rule is in effect. You are allowed to exagerate by 20% when consuming beverages and talking with friends. If you are trying to impress the dock babe, you are allowed to exagerate by a full 400% before your buddy is allowed to holler Bull$%it. :D

That estimator came frightnening close to my actual so I deleted it from my bookmarks!!!:D

you guys are right, this thing was right on the money!

are you guys able to use the fast V constant? I match well with the heavy V in a 272 baja.

My PRogression matches up well with the fast V setup. Of course I'm estimating weight and assuming that factory HP is reasonably correct.

I think those slip numbers are a little off. I am running a 26' cat with a dry weight of about 3850# and I am seeing about 13% slip at 104 on gps, but I was a little heavy on fuel. I think being light on fuel I might see 10%. A V bottom running less than 10% slip with a pleasure set up would not be very probable. My .02

I can't vouch for the slip numbers but plug these in and see what you get for the constant.

Weight 3900 (3100 dry plus 300lbs of gas, 325 lbs of passengers plus 175lbs gear)
HP 415
Speed (GPS) 82.3

I used the "fast V bottom" and got 82mph. You did not say what type boat you are running. That sounds a little fast for me in a V with that hp. I know a Mach 26' (cat) with an hp 500 (470 hp at the prop, not shaft) will only run low to mid 80's. Just my opinion but I don't think you will see to many V bottoms running less than 10%. If you are getting those kinds of speed with 415 pshp in a V that is damn good.:cool:

Actually I agree that you probably not see many V hull achieve these numbers. My boat is a Progression 24' V and experience tells me that few V hulls with equal power will run with this hull. Bottom is a notched transom, pad bottom deep Vee with a fair amount of rocker and all edges are sharp. Bottom is string straight for the last 6 feet. I don't know the X but it is higher than average.

Now I get sceptical when I run those numbers with the Whipple installed. Add 200 HP and about 100 lbs. I'm gonna need a lot of witnesses if that works out right.:eek: :eek: :eek:

98 mph is what I got. I agree you will need some witnesses:D . That is cat type speeds with that hp. Good luck!!:cool:

Cool, I only need 1550 more horsepower to get my Cafe Racer over a hundred.:D

Looks pretty acurate.....................unfortunately:(

Jim
 

I bet those are "lighter than air" horses also

V drive ski boat
 

the formula is not even close for a friends comp ski boat.

Would you account for the difference with the hull factor or HP loss with the drive?

The hull factor will be cignificantly different with a ski boat. the hull factor is a function of the wetted area of the boat.

The overall drag is due to three main contributors. The wetted surface and displacement of the hull, the aerodynamic drag ( ~10%) and the outdrive drag. Theoretically speaking ( physics) each contributor follows a power requirement as a function of velocity cubed (see my avatar) but the effect of lift from the aero and the pitch angle change as speed increase, significantly reducing the wetted area and thus the hull contributor. this overall efficiency (quasi hull efficiency) overall follows a squared function ( at least for cats, still looking to confirm for v hulls).

With a ski boat, the aero lift is insignificant, and the hull contribution is more significant that a fully planed deed V and a cat. (it's pitch never really changes, and it has a very large wetted area)

yada yada, i'm rambling....... the "hull factor" (ie quasi hull efficiency) is much different that a conventional V or cat

my $.03


Tomcat should be around any minute. He has good insite. He and I have had numerous discussions on this.

Tomcat. I guess I conceed on the fact that Cats follow an empirical v squared function ( my rationale is stated above. This allows me to still be kinda right regarding the physics of each contributor:D )

i would still like to have some before/after performance for deep V's, preferably with the same prop.

Yep, the lack of trim control has a big impact

Rambunctious - Are you going to the April 27th OSO meeting? I'm driving from north of Toronto and I hope to see you there!

I honestly don't know why power seems to be proportional to velocity squared in boats. In my business (industrial ventilation) we talk about the fan laws:

CFM varies directly with fan speed
Pressure varies as the square of fan speed
HP varies as the cube of fan speed

These "laws" are used to quickly estimate the new pressure and HP required when you want to increase the CFM through the system. You can see the cubed relationship with (fan) speed.

But these rules only apply where the pressure losses of each element in the system (elbows, contractions, expansions etc.) vary as the square of velocity of air through that element. That is the norm, but if you have an element, like a fabric filter to collect airborne contaminants, in which pressure loss varies directly with volume, you can't use these rules for a quick estimate; you have to recalculate system pressure losses.

This isn't a great analogy but is it possible that a planing hull has an element of drag which varies directly with speed? I guess that's what you're saying happens to the hull drag as wetted surface area decreases with speed.

The speed calculator on BAM's site is based on Mercury (squared)equations which seem to fit the data fairly well. I took these hull curve equations and added the ability to plot engine horsepower curves on top of them. This allows you to see how to prop the boat for your actual engine. The program has a prop optimizer button that selects the right pitch to make the engine curve cross the hull curve at maximum HP. See attached graph.

'Squared or cubed', 'Horsepower or Torque' Here we go again!
Tomcat and Rambunctious your threads and comments are great as usual.

Tom did I read that you have that book on tunnels? Are there any formulas in there into which we can plug in our boats specific numbers- tunnel size, shape, etc. and get a ?tunnel number? And then can we see what we need to do to our hulls to make them better. Could this book predict a perfect 35' cat. I thought the bool was a bit pricy.

tomcat, unfortunately i can't make the party because

We will be in VEGAS!!!

our boat club here at work will hold our 2nd annual poker run fund raiser this August . 25 boats our first year!! shooting for 50 this summer. Maybe you can tow the boat down. or come and watch the GLSC series race in Grand Haven labor day with some of us.