Spun prop - mark hub question

[bonzoscott]

I've been curious about the speed of my boat for quite a few years. It's a '90 model 18 foot - 4.3, 205 hp. I bought new, always garage kept and motor running condition is flawless. When new, would do 55 mph but over time, speed has been dropping. I have never taken it over 4200 rpm and speedo now shows ~40 mph. I couldn't figure why it drops that much speed with the same rpm. So i thought about a spun prop and marked the hub. I don't ever experience any rpm increases as if the prop was slipping like an auto clutch, but I marked the hub/prop anyway and checked it out. What would you say if the marks moved "slightly"? Just barely like 0.050. Has it slipped all the way around or do they move a bit anyway? I don't think it's spun, just don't know why I'd lose that much speed. I don't really care - not a speed demon, I'd just like to make sure everything is in order. I do put quite a load on it slalom skiing. Just looking to see if others have experience with this.

 

[GA_Boater]

Re: Spun prop - mark hub question

5 hundredths of an inch isn't slipping. Odd of the marks spinning around are pretty slim. Does the boat seem to tow heavier? Could there water in the foam? What is the WOT RPM and does the motor still get there?

 

[V153]

Re: Spun prop - mark hub question

Have you checked speed via GPS? Mebbe yer speedo took a dump. Ya it's unlikely your hub spun 360 degrees & 5 hundredths ...

 

[smokeonthewater]

Re: Spun prop - mark hub question

if the hub was slipping the marks would be in a different place every time you looked at em... have you been beating the prop up? that would cause you to lose speed

have you been eating at the buffet every day? LOL J/K

 

[bonzoscott]

Re: Spun prop - mark hub question

I agree with some comments. Unlikely it spun all the way around. I have never taken it completely to WOT. It is absolutely screaming at 4200 and there's not a whole lot of throttle left. The prop is the original, never hit anything and appears good. I thought about the GPS thing but I'm not tech savy with electronics, smart phones and the like. I'll find someone with a GPS and check next year. Again, just looking for others experience. Do speedos lose speed? Maybe there's a worn diaphragm in it or something. Do props "lose" pitch after 22 years? And it's not the buffet.. I'm actually 30 lbs lighter than years ago when the boat would do 50+. No heavy women either...
Oh yeah.. I'm sure it's not water logged. Been garage kept since day 1. Thanks for the replies!

 

[Sea Rider]

Re: Spun prop - mark hub question

I've seen boats with water logged hulls on firsts outings which owner didn't know a thing, not even having a water drain valve on hull, so garage keeping doesn't say much. With boat sitting on trailer, make a trailer wheely, open water drain valve and check. You should go for a wot spin with you alone and check max achieved rpm and compare them to engine's wot factory working parameters to know if under or over propped, once you know it, could go for a prop pitch rpm optimization.-

Happy Boating

 

[Texasmark]

Re: Spun prop - mark hub question

Water logging or dirty hull. Doesn't take much contamination on the hull to really make a dent in your mpg. Agree with the comments about the spun prop.

Mark

 

[Sea Rider]

Re: Spun prop - mark hub question

Oooops wrong wording, a floded internal hull will make a difference regarding hole shot, less top rpm at wot + having spun prop or not, a matter to unscrew water darin valve and check. Water condensation inside hull does mean things when boating.

Happy Boating

 

[81 Checkmate]

Re: Spun prop - mark hub question

Just asking......when was the last time you tuned up the motor...Also ever do a compression test. Could be your motor is getting a little tired and it's not putting out the HP that it had when new. 23 yrs old now?

 

[Bondo]

Re: Spun prop - mark hub question

I agree with some comments. Unlikely it spun all the way around. I have never taken it completely to WOT. It is absolutely screaming at 4200 and there's not a whole lot of throttle left. The prop is the original, never hit anything and appears good. I thought about the GPS thing but I'm not tech savy with electronics, smart phones and the like. I'll find someone with a GPS and check next year. Again, just looking for others experience. Do speedos lose speed? Maybe there's a worn diaphragm in it or something. Do props "lose" pitch after 22 years? And it's not the buffet.. I'm actually 30 lbs lighter than years ago when the boat would do 50+. No heavy women either...
Oh yeah.. I'm sure it's not water logged. Been garage kept since day 1. Thanks for the replies!

Ayuh,.... So yer sayin' this is a pristine hull that sits on a wagon in the garage all the time,..??..

Along with Checkmate's question 'bout motor service,.....

Have ya peeked at the Runnin' Surfaces of the hull, with a straight edge lately,..??

just throwin' that out there,....

Is the wagon bunks or rollers,..??

 

[bonzoscott]

Re: Spun prop - mark hub question

I'm quite sure no water logging, though not 100%. Boat is garage kept except 4-6 hour trips on nice days. But I guess the wood is a sponge. Maybe I'll weigh it. Bilge is nearly spotless. The hull is spotless, waxed annually, but have not checked with a straight edge - I will though. It has been quite a few years, probably 8-10 years since a tune up. I don't have as many trips as my younger years. But my simple mind tells me: If it runs seemingly perfect and very responsive all the way to 4200 rpm, new points won't give ya more speed at the same RPM. Maybe I'm not understanding basic principles. Again, I have no problem with my use of it. I'm just trying to make sure I get 40 years out of and be proactive. I take vitals of my motor annually and if everything is in order - drive on. I'll take advise and check these things out in the spring.
Last question - bunks

 

[Chris1956]

Re: Spun prop - mark hub question

55MPH at less than WOT for that motor/boat combo is real good. I had a similar package 18' SR bowrider with the 4.3LX motor. 42MPH was about the best I could do at about the same RPM you tried.

Your speed was tested when new and now with a GPS? If not I can adjust your speedo to show any speed you want. One shot with an icepick, and you will be hitting 55MPH again.

 

[limitout]

Re: Spun prop - mark hub question

something has been bugging me since it was commented on in this thread.

the OP said he marked the prop and it moved "just a tad" (my words) but is this normal? do props move at times if you hit something or just from being under load over time?

question is, shouldn't you have no movement at all or if you do such as the OP noticed, shouldn't you need to find out "why" it moved, even though it was such a small amount? is a tiny amount of movement considered "normal"?

 

[GA_Boater]

Re: Spun prop - mark hub question

It's a rubber hub meant to move for shock loading when shifting gear in/out of gear. I think .005 of an inch is just the margin of measurement error. The prop didn't move. Most likely the boat needs a tune up after eight to ten years.

 

[V153]

Re: Spun prop - mark hub question

Most likely the boat needs a tune up after eight to ten years.

Ya, I'd say yer 'bout due ...

 

[GA_Boater]

Re: Spun prop - mark hub question

Ya, I'd say yer 'bout due ...

Ain't mine. :facepalm:

 

[ken51k]

Re: Spun prop - mark hub question

How are you getting the speed of the boat, with the boats speedometer or a good GPS? Boat speedometers are not very good. It could be just that the speedometer is getting weak or you got dirt in the pickup tube.

 

[steelespike]

Re: Spun prop - mark hub question

Any spun hub I have seen; The motor revs and the boat slows. Reduce throttle and prop works until yopu reach too much power again.Sometimes the hub may work for a while from the heat.But will spin again. To my experience when it slips its all or nothing.
It doesn't cause minor rev problems.
The boat is 23 years old. I think a tune up with a compression check is in order.
Is the bottom clean and smooth? Does the throttle open all the way?Is the timing set right and advancing as it should?
Verify the speed with a GPS and rent or borrow a shop tach to check the rpm.

 

[bonzoscott]

Re: Spun prop - mark hub question

Any spun hub I have seen; The motor revs and the boat slows. Reduce throttle and prop works until yopu reach too much power again.Sometimes the hub may work for a while from the heat.But will spin again. To my experience when it slips its all or nothing.
It doesn't cause minor rev problems.
The boat is 23 years old. I think a tune up with a compression check is in order.
Is the bottom clean and smooth? Does the throttle open all the way?Is the timing set right and advancing as it should?
Verify the speed with a GPS and rent or borrow a shop tach to check the rpm.

Thanks Steel Spike - limitout's question was originally why I posted. The mark moved "a bit". Running the boat hard never had the sensation of slipping. Vitals are checked annually, meaning compression - spark - timing and gets new plugs every other year due to fogging. The boat jerks a 200 lbs slalom skier right out - plenty of torque. Finish on gelcoat like new. GPS is my thing, I think the speedo is off. It sounds like it's ready to blow at 4200 RPM so I never considered an alternate tach. Chris1956 mentioned 42 MPH best he did with a similar set up -heck, mine coulda been off from day one. I liked his idea bout the ice pic too! This is the prop forum so I thought I see what folks have experienced. I don't do smart phones and GPS' but I'll get someone to help me out. It's just everything seems to be in order (minus ~10 mph). While I sure will take everybody's advice and do a full tune up - answer me this: If you have spec'd out spark, no stumbling or missing and achieving darn near operating RPM range, why tune? The RPM translates to the prop. How would a "Tune Up" bring the speed up at the same RPM? It's gotta be the speedo or some difference between the crank and the prop (or weight). Appreciate the feed back...

 

[steelespike]

Re: Spun prop - mark hub question

Ok, so we are saying the rpm remains at 4200 but speed is down 15 mph.
First of all I don't think you can do 55 at 4200. Using a 1.62 gear ratio and a 21" pitch you get about 45 at 12% slip.
with a 23" prop you would get about 49. You mention Screaming at 4200.Could indicate the tach is wrong.
Perhaps the prop is undersized as well.
Do you know your gear ratio?Usually on the out drive. What size prop are you running?
I didn't see these mentioned.