HELP ... high RPM'S low speed

[DennisG01]

My favorite is when it says we are doing 4 knots when tied to the dock.

Eric, is the boat faced into the current?

For what it's worth, GPS is not entirely accurate, either, depending on if you are going with the current or against the current as compared to a slack tide. If you want a true speed, you should make two runs 180* to each other and split the difference.

 

[against the wind]

thank u uno eric .. I understand exactly what ur saying , but a 200 yamaha carbed at wot 4800 rpms should be running more than 28 mph.. i am waiting to have the vessel surveyed but in the mean time im trying to get this problem resolved. im thinking low compression, what do u think :-|

 

[enfish]

Eric, is the boat faced into the current?.

Ha! No, it's totally random as far as I can tell. I don't think we ever have 4 knots of current at the launch ramp, even on extreme tide swings. Unless, of course, we're having a tsunami event ;-)

https://youtu.be/ropC7NzcVlI

[youtube]https://youtu.be/ropC7NzcVlI[/youtube]

 

[enfish]

thank u uno eric .. I understand exactly what ur saying , but a 200 yamaha carbed at wot 4800 rpms should be running more than 28 mph.. i am waiting to have the vessel surveyed but in the mean time im trying to get this problem resolved. im thinking low compression, what do u think :-|

Yes, it should be faster than 28 mph, but probably not by too much. At 4800 RPM, I'd expect my boat to be 32-33 mph.

I don't think it has anything do to with the engine though. Unless the drive train is slipping when it shouldn't be, the prop is going to be turning an RPM directly related to the engine RPM (differing by, I believe 1.86, which is the gear ratio for that motor). So if the motor is turning at 4800 RPM's, the prop will be spinning at 2580 RPM's, which will push the boat at whatever speed it can for the conditions, regardless of the compression of the motor. If the motor had bad compression, then it would have a harder time achieving higher RPM's for a given load. But since you can get close to what the rated WOT RPM is for that motor, I don't think that's a problem.

The only thing that can cause the hull to move slower than expected, since a 15x17 prop is pretty close to the expected correct prop for the rig, is excess drag, which would cause excess prop slippage in the water. Excess drag can be caused by a number of things...

1. Bad hydrodynamics (i.e. the bottom of the hull is not smooth)
2. Excess weight in the boat
3. Motor poorly trimmed
4. Prop hub is spun (so the propeller blades aren't spinning at the same rate as the prop shaft)

...to name a few.

Since we've got the same hull, same prop, and close to the same motor (which should have the same gear ratio), we can do some comparisons using this prop slip calculator:

http://www.go-fast.com/Prop_Slip_Calculator.htm

So, using the middle of my range of numbers, here are my prop slippage %

3600 RPM = 32%
4000 RPM = 31%
4200 RPM = 26%
4400 RPM = 20%
5300 RPM = 24%

My numbers are a little bit skewed because I can only run above 4200 RPM's in glassy conditions, whereas at 4200 RPM is typically where I like to be able to cruise in less than ideal conditions. So my prop slip calculations for 4400 and 5300 are on dead flat calm water, but the other RPM calculations are from less than glassy conditions. I also have trim tabs, which will affect the numbers slightly, too.

A lot of people say those slippage numbers are too high, but those people must only boat on flat glass lakes with very light boats. From my experience, which is mostly all fishing offshore in small boats, those numbers are typical for a number of different boats. Theoretical top speed for my boat at 5300 RPM WOT with 0% slippage is 46 mph.

With your numbers, you've thrown on 28 mph at 4800 RPM and 28 mph at 5000 RPM. Using both of those for calculation:

4800 RPM = 33%
5000 RPM = 35%

Those are, not unexpectedly, pretty high numbers, but not absolutely horrible. So there's something about that boat that's either causing excessive drag, or the prop hub is spun. Though usually with the latter, it won't be long until the boat won't be able to get on plane anymore with the spun prop.

Just my (probably more than) 2 cents!

 

[against the wind]

your numbers are spot on match.. a older man owns the boat and i think he really does not know to much about how the trim tabs work .. when i took her out i could not notice any slippage in the prop [how can i tell] she ran great no hesatation no shaking ran strong.. He did tell me though when his mechanic changed the plugs this spring he just took them out of the box, and installed them did not gap them.. i told him i didnt know if they came pre gapped or not.. do u think that is a possiblity for the low speed thanks

 

[enfish]

your numbers are spot on match.. a older man owns the boat and i think he really does not know to much about how the trim tabs work .. when i took her out i could not notice any slippage in the prop [how can i tell] she ran great no hesatation no shaking ran strong.. He did tell me though when his mechanic changed the plugs this spring he just took them out of the box, and installed them did not gap them.. i told him i didnt know if they came pre gapped or not.. do u think that is a possiblity for the low speed thanks

It's never a bad idea to get the motor and boat surveyed before making a purchase, but from what you've described, I don't think it's the motor causing the slow speeds. Are you able to reach the rated RPM at WOT?

My guess is either rough bottom paint or heavy hull (due to how much stuff is on the boat, or there's always a possibility the foam flotation is waterlogged). I can tell you my boat is a couple knots slower when I've got 250 lbs of water in the bait tank on the deck, 500 lbs of gasoline in the main tank, and an additional 180 lbs of gasoline in jerry cans, and 40 lbs of ice on board than when I don't have any of that extra stuff and only half a tank of gas. But, it rides a little bit nicer when it's super heavy... speed and fuel economy just suffer a little bit.

Prop slippage is not something you can really feel. It can only be calculated. With a 17 inch pitch prop, 0% slippage would mean the boat would move forward 17 inches for every 1 rotation of the prop. Analogous to driving a screw into a piece of wood... there's 0% slippage when you do that unless you start stripping out the hole. But since water isn't solid, the prop simultaneously pushes some water backward while pushing the boat forward. The amount of water being pushed backwards vs. the amount the boat moves forward is prop slippage. If you were tied off to the dock and put the motor in gear, then you'd have 100% slippage since the prop would only be pushing water backwards and not propelling the boat forward.

 

[DennisG01]

Eric - that guy wasn't making much headway, was he! But I bet his speedo said he was doing 10mph! :mrgreen: Good video.

AgainstTW - there's lot's of things that it "could" be. But that's just it - it's just guessing at this point. Have you taken the boat back out and confirmed that it's being trimmed properly (mentioned above)? Also, take out any unnecessary weight for this trip.

 

[against the wind]

thank u guys for all the help !! mechcanic not coming till next week however im trying to get more educated while i wait for him.. i diff understand the slippage ratio and makes perfect sense ... thank u much. when i drove her we had 3/4 tank of gas calm water,,and total weigh was btwn the both of us was about 400 lb thats it.. owner did have a compression test 80 to 85 psi in each cylinder.. Is that good .. i thought it should have been high than that 110 to 125 correct me if im wrong thank u

 

[against the wind]

i never had trim tabs on my other boat, i dont know how to trim it out

 

[Lt.Mike]

I'd say its a spun prop.
When mine went I could only get about 25-28mph out of it too.