Determining speed

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gouran01

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A bored day got me thinking, I've removed about 100LBs of excess weight from my boat over time which is roughly 15%. Now with the HP to weight ratio being 15% better should I expect about 15% more speed? Anyone ever saw a formula for this?
 
That seems to give very high speed predictions...maybe I am doing it wrong, but I don't see how.
 
Thanks OLD...that is a nice addition to my LINKS. As far as accuracy... I put in a theoretical 1000 lbs with a 40 hp engine and a "hull" of 180 (whatever that meant.). The answer was 36 mph, which, I agree, sounds really high.

However, I then subtracted 150 lbs (now 850 lbs) and got a speed of 39 mph or a 3 mph difference. That difference calculation sounds more realistic, but who really knows without an on-the-water test. R
 
I calculated everything with my existing speed. figured 2000lbs loaded, my hp, and a hull factor of 160, to get what my known loaded (boat, motor, gear, 2 people, gas, ect..) MPH is, then added HP to see what happens. Doing it like this seemed to be fairly accurate.
 
That really is a great link. I'm curious to know how to figure different hull factors though and how one would go about knowing what their hull factor is.
 
it was dead accurate almost for my Stingray and my freinds Whaler. I havent got the Jon set up right to see its top speed but the results on it seem right in the ballpark
 
I put in all original info pre-build/removal and then adjusted hull factor until I got to the right speed in the calculation. Being a 17.5 foot "bassboat" I assumed the hull factor would be around the 170 range but hadda drop it all the way to 125. From there I made the weight and HP adjustments. Still playing with it though! nice tool!
 
Speed was fast when I calculated using that website. If you have a phone that is able to download apps you can download a speedometer that uses gps for free. Search dashboard if you have a iPhone it's great!
 
Yeah my 9.5 Johnson isn't going to get me over 35 mph...so either what I entered is wrong or their calculations are weird.
 
Jdholmes said:
Yeah my 9.5 Johnson isn't going to get me over 35 mph...so either what I entered is wrong or their calculations are weird.


Perhaps you are aware that you can work that formula backwards. If one were to do that on your boat using 35 mph, your boat woudl weigh 229 lbs in the aggergate. I am jealous that you could build that nice rig and have it, you, motor, gas, decking and everything weigh that little. Good job.
 
oldandugly said:
Jdholmes said:
Yeah my 9.5 Johnson isn't going to get me over 35 mph...so either what I entered is wrong or their calculations are weird.


Perhaps you are aware that you can work that formula backwards. If one were to do that on your boat using 35 mph, your boat woudl weigh 229 lbs in the aggergate. I am jealous that you could build that nice rig and have it, you, motor, gas, decking and everything weigh that little. Good job.

Sorry to the literate types, forgot about spell check.
 
I can't remember what I did the first time...

I tried it at around 500 lbs this time...I figured the boat is only a couple hundred, my motor is like 70lbs tops and I only weigh 140 lbs...the rest of my gear wouldn't take more than even 50 lbs beyond that...soooooo....I still think its off cause that put me at about 25 mph....maybe I am off on the weight of the tin?
 
I think hull factor is where yall may be going wrong

I used the 218 for my stringray and the dry weight plus me, gas and my gear and it was dead on at 48MPH

I would think using a hull factor of 150 would be accurate for our tin boats, and that also yielded the correct speed for mine
 

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