Top Speed?

[PITBoat]

I addressed this at the tail end of another thread I started, but it was kind of buried...

I suppose that number rather varies depending on the particular boat?

Mine:

'88 Renken 2000 Classic, 19.5 or 20' ski boat
4.3L (175hp?)
14 1/2 x 19 3-bladed prop
Don't know the weight...
Has a whale tail thing over the prop that I believe is aftermarket, but don't know

Apparent top speed as currently configured - a little over 40mph.

Does this sound about right for this engine/prop combo, or is that really not enough info.?

 

[Scott Danforth]

and what RPM are you spinning?

you provided about half the info needed

was the speed from a GPS or the speedo>

the whale tail thing will slow down your top speed

 

[PITBoat]

RPM - 5,500 or so? I don't trust it. That's from a factory tach that comes in and out (connections or something - I made sure they were tight and fiddled with a slotted arrow on the back... and it didn't work at all after that). I've replaced it, but need to take the new one out on the water. It shows a lower idle and I see no reason it won't read right.

Speed is from a GPS.

Short of getting back on the lake and testing the new tach, I'm just searching for the possible answer ahead of time; i.e., is it really turning 5,500 and I need a different prop, or no, it's turning the advertised WOT rpm, and 40mph is about all it's got?

I'm fine with that, I just want to be able to run there some more w/o worrying about over-revving.

 

[vasy]

the whale tail thing will slow down your top speed

Why would it?
Boat should run with that thing out of the water on plane.

 

[Scott Danforth]

Why would it?
Boat should run with that thing out of the water on plane.

They add drag and do not get out of the water

 

[vasy]

They add drag and do not get out of the water

Some chat on this topic: https://forums.iboats.com/forum/eng...96-whale-tails-on-a-motor-does-it-really-help

Seems some say the loose a lot of speed, some don't notice too much, must really depend on the boat.
So if you are getting just over 40, it is not too bad but remove the fin and see what you get and how the boat planes and after make the call.

 

[HT32BSX115]

I addressed this at the tail end of another thread I started, but it was kind of buried...

I suppose that number rather varies depending on the particular boat?

Mine:

'88 Renken 2000 Classic, 19.5 or 20' ski boat
4.3L (175hp?)
14 1/2 x 19 3-bladed prop
Don't know the weight...
Has a whale tail thing over the prop that I believe is aftermarket, but don't know

Apparent top speed as currently configured - a little over 40mph.

Does this sound about right for this engine/prop combo, or is that really not enough info.?

Scott's right. Need more info. Specifically: Accurate RPM, accurate speed, drive ratio and prop pitch.

So with your info (assuming a 1.68:1 drive).....19p prop, and using 5500 RPM (which is probably way high)

It would go about 50 mph @ 5500rpm (assuming the engine was capable)

Pretty unlikely a 4.3L V-6 would do it in a boat that size

If everything is running "ok", and you're doing 40 mph (verified) you're probably closer to 4500 RPM

That's assuming 15% slip and using http://www.go-fast.com/Prop_Slip_Calculator.htm

Using 5500 rpm and 40 mph, you would have 32% slip which is very unlikely.

Cheers,

Rick

Let me also add that in 30 years (unless you bought that boat new) someone could have changed the drive (many times) The drive *could* be a 2.00:1 ratio (as removed from a 2.3L 4-banger)

That could very well result in your numbers if the RPM is accurate.

This is why you need to verify your tach, speedometer, and drive ratio. Until you know for sure, you're wasting your time trying to figure out top speed or other performance problems

 

[PITBoat]

Got a chance to try the new tach out today. Heading back to the dock with the wind (8 or 10mph?) behind us I noted:

29mph at 3,000rpm
40mph at 4,000rpm, and
42mph at 4,150rpm (all out, max rpm)

That max rpm is much more believable. A little below what it should be though.

I think I need to adjust the trim tab. It is pretty much centered, but the boat wants to turn left slightly at higher speeds if I let go of the wheel. I attributed it initially to a quartering headwind, but I don't think that was why.

 

[HT32BSX115]

Got a chance to try the new tach out today. Heading back to the dock with the wind (8 or 10mph?) behind us I noted:

29mph at 3,000rpm
40mph at 4,000rpm, and
42mph at 4,150rpm (all out, max rpm)

That max rpm is much more believable. A little below what it should be though.

I think I need to adjust the trim tab. It is pretty much centered, but the boat wants to turn left slightly at higher speeds if I let go of the wheel. I attributed it initially to a quartering headwind, but I don't think that was why.

Yup, mo-better numbers! (assuming you've verified your speedometer)

If your engine is healthy (good compression, tuned up, carb, timing etc) you might give a 17p stainless steel prop a try.

Also, make sure you are not overly loaded (or lightly loaded), balanced (I.E. not leaning) and you're trimmed (drive angle) for max speed at WOT.

You may even get a small improvement in top speed with the slightly lower pitch prop (because the engine develops more power at higher RPM) , but it will more likely come out of the hole better.

In any case, you want to select a prop that will put you at or near MAX rated RPM at the load you'll operate at most of the time (people, gear, fuel level, etc)

 

[PITBoat]

It won't tolerate a whole lot of Up trim, but I gave it the 3 or 4 momentary shots that it will. When it starts to bounce a little I back off it.

It runs pretty well, but not perfectly - I think the exhaust stinks more than it should, and there is a small dead spot early on in the throttle range. I'm guessing that's carburetion, but I haven't checked anything (plugs, points, etc.). I don't really detect any misfiring or the like though. I'm unpacking all of this a little at a time. I have a decent knowledge of cars but the boat-specific stuff is new to me.

There was myself and 4 kids yesterday, and maybe a little less than 1/2 a tank of gas (30gal capacity I'm told). It was probably a load representative of what I'll normally carry. I've been seeing stuff about water logging on here - something else for me to worry about. For all I can tell it sits pretty high though, and doesn't take on water in the bilge, so hopefully I'm not carrying any extra weight of that sort.

I may try that prop suggestion next season. The current prop has a little damage on one blade (a displacement of maybe 1/8", for 1" or so along the leading edge). Don't know how much negative effect that may be having. May take off the whale tail too just to see what happens.