Posted Speeds on Boats

[bajaunderground]

A good buddy of mine just ordered a 2014 Interceptor 24SST. While I have no doubts it's a nice enough boat, what concerns me is the posted speeds Mercruiser is putting on there website.

The Boat my buddy ordered (his is all yellow though):
Specs: 24'5"
Beam 8'4"
Engine: 5.7L 300HP
Drive: Mercrusier Bravo 1 (1.82 ratio for altitude)
Weight:3,700lbs


THE TEST POSTED ON MERCURY'S SITE:
The engine/set-up is the same as the Interceptor in a similar modeled Caravelle (Maker of Interceptors)

Engine Test | Mercury Marine

They're stating a tested speed of 52.1MPH @ 5,200 with a 15.5" x 17P prop...The max RPM's is 4,800-5,200. SO crunching the numbers to achieve that speed would mean a drag coefficient of -13% (impossible) with a 1.65. The only way to achieve these numbers are with a 1.5 Bravo 1...and this with a drag coefficient of 6% (still I feel impossible given the size/weight of the boat tested?!)

Any insight? I'm not trying to bash Mercrusier nor dispute their results...call me skeptical? I'm just trying to get an accurate idea of a realistic speed for my buddy!?

SIDE NOTE: The boat he sold was a 1999 Stroker Bass boat with a 300 Mariner Supermag...84.5mph on GPS with a full tank and two oversized dudes...he and I!


~Brett

 

[tpenfield]

Re: Posted Speeds on Boats

It would be interesting to see the math that calculates the drag coefficient that you speak of . . .

The boat is fairly light at 3700 lbs dry for a 24 footer, so probably enough engine. I would expect something in the 50 mph range.

 

[craze1cars]

Re: Posted Speeds on Boats

oops...double-post...

 

[craze1cars]

Re: Posted Speeds on Boats

6 or 7 % slip is a good number, and not impossible to achieve.

I've owed speedboats in the past that posted up legitimate 0 to 4 % slip numbers when run thru calculator. Obviously this is mathematically impossible, but the calculators are not exact science when you introduce cupping. Several prop slip calculators will advise you to add "about 1" to pitch while running calculations with a cupped propeller.

Why are you assuming they did test with 1.65 ratio drive? Their test doesn't disclose drive ratio.

I has a similar HP/size motor in a 21 foot Stingray recently. Ran a legitimate 60 mph GPS. If the boat was a 24 footer instead, I think 52 is a very reasonable guess in perfect conditions/light load.

Your buddy will never see it at altitude.

 

[drewactual]

Re: Posted Speeds on Boats

one other thing to bear in mind: if the prop in question has an aggressive rake and a dramatic progressive pitch, it is absolutely possible to hit that speed at 5200rpm...

progressive prop blades spec out at the average pitch NUMBER from end to end, not off the middle of the blade... it may (and possibly does) swing as much as 1.5~2" of pitch, which would allow (given, in absolutely perfect conditions) a hole shot with water slipping of the leading (splashing) edge at around 15.5" pitch, and once bite is established and on plane- trimmed properly- have water slipping off the trailing edge of the blade where the pitch (just prior to the cup) could be as high as 19" of pitch...

the specifications supplied by makers only tell a part of the story.. the geometry tells the rest of the tale, but when that is introduced it can't be properly discussed without involving hull design, weight, distribution of weight, dead rise, ect... that's why this crap ain't science, but art, instead...


edited to add: if there was a 5% slip, which is pretty tight, and @ maximum rated rpm of 5200, the prop would have to be 20.5"p at the trailing edge to accomplish that speed with that gearbox.. it could do it with a 19p trailing edge with a 1.63:1 gearbox and the same slip... just because the 350mag is rated to 5200rpm maximum RPM doesn't mean the rev limiter kicks in there, a matter of fact they could push upwards of 200rpm over that- meaning, the engine could have produced a WOT run of 5300~5400rpm on a progressive prop and nailed that speed with relative comfort...

 

[bajaunderground]

Re: Posted Speeds on Boats

one other thing to bear in mind: if the prop in question has an aggressive rake and a dramatic progressive pitch, it is absolutely possible to hit that speed at 5200rpm...

progressive prop blades spec out at the average pitch NUMBER from end to end, not off the middle of the blade... it may (and possibly does) swing as much as 1.5~2" of pitch, which would allow (given, in absolutely perfect conditions) a hole shot with water slipping of the leading (splashing) edge at around 15.5" pitch, and once bite is established and on plane- trimmed properly- have water slipping off the trailing edge of the blade where the pitch (just prior to the cup) could be as high as 19" of pitch...

the specifications supplied by makers only tell a part of the story.. the geometry tells the rest of the tale, but when that is introduced it can't be properly discussed without involving hull design, weight, distribution of weight, dead rise, ect... that's why this crap ain't science, but art, instead...


edited to add: if there was a 5% slip, which is pretty tight, and @ maximum rated rpm of 5200, the prop would have to be 20.5"p at the trailing edge to accomplish that speed with that gearbox.. it could do it with a 19p trailing edge with a 1.63:1 gearbox and the same slip... just because the 350mag is rated to 5200rpm maximum RPM doesn't mean the rev limiter kicks in there, a matter of fact they could push upwards of 200rpm over that- meaning, the engine could have produced a WOT run of 5300~5400rpm on a progressive prop and nailed that speed with relative comfort...

What you say makes complete sense...

I'm still skeptical of the results! I mean I ran my Baja with a * x 21P SS prop (forget brand and whatnot) the boat, with motor, was 1,600lbs and I could just reach 65mph (GPS) with a 200hp Mercury spinning 6,000rpms and a 1.82 gearing...doing the numbers I had a drag of 5%, which seems reasonable for that style of boat...at 4,300' elev.

I just think for that style/weight of boat those kind of numbers are a stretch...now if they run in excess of the listed RPM's (Which the test had them posted for the speed) then I'd say it's plausible?

I get that props/hulls/trim/slip/progressive pitch is more an art form that and "exact" science...but I belong to other websites (screamandfly.com and checkmate-boats.com) both of which have a slew of high-performance boats (OB and I/B) and not many people with a 300HP 5.7L motors and in similar sized boats are seeing those numbers in the real world...let alone at altitude! Most are running 496Mags or 502mags...

By no means am I saying the boat can't achieve these numbers...having grown up around boats my whole life, I've never seen those numbers duplicated in the real world...

~Brett