Coast Guard Rating Plate

[FlyBoyMark]

Re: Coast Guard Rating Plate

Precisely what I and others have told JM who started this thread, his boat would do 60 MPH on a 115 HP = to an old 125 HP of its day 30 years ago which it was NEVER designed to do. He WANTS a new boat but has real good deal on NEW 115 NOW if he can swing it and TEMPORARILY wants to mount it on his existing old boat HIMSELF. But MY whole conversation is yes he can do it in this state but watch out for:<br /><br />327.33(1) when a vessel is operated in a grossly overloaded or overpowered condition.<br /><br />And the only thing that is going to draw attention is "Appearance".

 

[csmithrc]

Re: Coast Guard Rating Plate

HI all, I have A 1977 Glassmaster (17.5ft) that i bought from the original owner.The plate on it says 135 max hp.It has a 1977 140hp Evinrude on it(original motor).But even with this motor the max speed i have gotten is 38 mph(using trim).This is with engine turning 5500 rpm and have tried ss and alum prop(both 12 3/4 x 21).Does this boat sound underpowered to anyone?

 

[FlyBoyMark]

Re: Coast Guard Rating Plate

Yours is not but the original poster of this thread, his boat was a 30 year old 15' lite weight tri hull that with an old 50 hp head rated engine would do 30 MPH or more. Basicly again, he'd be almost trippling the HP with a new 115 HP 'cause its rated at the head probably 125 hp or more. The old placards are based on HEAD HP rating and not at the prop. Your old engine is probably equal to a 130 HP of today approxamately.

 

[Franki]

Re: Coast Guard Rating Plate

couple of points..<br /><br />1. I have a table just like the one in that photo,, and a 6hp outboard.. I'm gonna try it and get a gps to tell me how fast I go.. sounds like a good laugh.<br />2. How long did you want to keep that big motor on the small boat?<br />3. Remove the HP rating off the motor and don't go over half or 3/4 throttle.<br />4. put it on the transom of the boat.. then put an additional 50-60kg on the motor.. does the transom flex much? if so, don't leave it there.<br />5. move everything you can up to the front of the boat.. .<br />6. Its an old boat, probably didnt' handle that well to start with, with the extra weight, it may handle worse,, particularly since the extra weight is high up.. cornering at speed can easily result in a rollover.<br /><br />Take it easy, hide the numbers, keep the boat lookin reasonably level and neat.. and for gods sake, resist the urge to "see how fast it goes" you could kill yourself or someone else..<br /><br />but if its only for a number of months.. and you don't push it.. you'll probably be alrigth..<br /><br />Thats a technical analysis.. not legal.. thats not my area..<br /><br />PS, how fast do you rekkon an 8x6 table will go with a 6hp outboard on it??? I suspect the handling will be pretty sus too... <br /><br />rgds<br /><br />Frank

 

[FlyBoyMark]

Re: Coast Guard Rating Plate

Divide the rated RPM@HP of the engine by the lower unit ratio (ex: 2:1 so divide, lets say, 5000rpm by 2) Multiply that number by the PITCH of the prop(will = inches per minute)Multiply that answer by 60 (will = inches per hour)divide that figure by 12 (will = feet per hour)Divide that figure by 5280 (will = theoreticle miles per hour)<br />To calculate slipage; multiply MPH by .92, this is for a fairly efficient hull such as a lite large flat bottom hull. The final answer should be real close + or - 1 mph.

 

[csmithrc]

Re: Coast Guard Rating Plate

Flyboy according to that calculation my boat should go over 45mph.RPM(5000 divide by 2=2500x21(pitch)=52500x60divide by 12divide by 5280x .92=45.73 Am i missing something here.Thanks

 

[FlyBoyMark]

Re: Coast Guard Rating Plate

Certain hulls are less effecient than others. You can lose up to 20% or more on planing hulls. .92= 92% effecientcy. My little aluminum V-hull bass boat does 38 MPH with a 40 HP merc or .91 eff. As the vee gets deeper, the weight increases, aerodynamic drag or head wind increases the slippage increases. Hull condition has bearing on this too as does the prop. Stock aluminum props are the best props for efficientcy but they lack durability. With my little boat every thing is brand new.......so this is why I get such sparkling performance from a small motor. If your hitting 38 MPH, actually thats not bad for an old setup like yours. Jacking the motor also helps improve eff. too.(maybe 1-2 MPH, gave me 1.5 MPH on my setup)

 

[DJ]

Re: Coast Guard Rating Plate

FlyBoyMark,<br /><br />Not picking, but:<br /><br />The deeper the "V" or deadrise does not increase the "weight" of the rig, however it certainly can increase the "wetted" surface.<br /><br />The term "displacement" in marine terms TYPICALLY (not always) means weight.<br /><br />Just like your airplanes, certain wing designs certainly can add lift, but sometimes at a price.

 

[FlyBoyMark]

Re: Coast Guard Rating Plate

You took it out of context, that is exactly what I meant.......The variables are: weight, hull shape (hydrodynamic drag), prop, lower unit position, hull condition, wind and aerodynamic drag. Did I miss anything?

 

[jm]

Re: Coast Guard Rating Plate

I appreciate everyone's input. Here is what I have done. I found an 80's 70HP outboard which is 5hp under what the boat is rated for. I'd rather be safe than sorry and as far as purchasing a larger boat, if the 70hp seems to weak i'll just sell it and buy a larger one when I have the larger boat.