Newbie Prop and WOT question

[svkb]

Hello all,

I'm brand new here and new to boating. So far I'm loving my new "sport"! I have a concern about the boat that we purchased last fall though and I'm hoping you guys can give me some guidance. I'm a complete newbie when it comes to boating and anything related to it so you I hope you guys don't find my questions too ridiculous. I have searched the forums but there is just so much information and I'm not really sure exactly what to search for to get the answers I'm looking for, so please bear with me.

We purchased a 1991 SeaRay 170 open bow with a 3.0 litre Mercruiser. The engine only has 160 hours on it. My question has to do with speed and pitch. I just checked my prop and it is stamped 15.25 x 15P, so as I understand that means it is 15.25 inch diameter with a 15 inch pitch. We had the prop repaired over winter. We have taken it out a couple of times this year and have achieved the following results.

Top speed with 2 adults and a small amount of gear - 34 mph (confirmed by GPS)
RPM at WOT - 3900 - 3950 rpm (confirmed with a timing light)

Now correct me if I'm wrong, but isn't that speed impossible given the pitch of the prop and the rpm that my engine is running?

With a 15" pitch shouldn't I expect that engine to be screaming? I've been told that I should run a 17" pitch to get 4600 rpm WOT by my local boat shop.

Some additional information:

The gas that WAS in the tank was at LEAST 2 years old (possibly older) and the tank was 3/4 full when we purchased it. Since then we filled up and burned 1/2 tank, then I put in a can of seafoam and filled up with premium and have burned through 3/4 of a tank.

I have confirmed that the throttle linkages are opening the butterflies all the way.

Sorry there are a lot of questions in there, but I'm hoping that you guys can give me some guidance.

 

[tpenfield]

Re: Newbie Prop and WOT question

The engine RPM seems low . . . you need to factor in the gear ratio of the outdrive when calculating how fast a certain pitch prop should make the boat go (less prop slippage, of course) . So, it is hard to say . . . do you have any history on the boat as to what speed and RPM it has achieved in the past?

 

[Bob_VT]

Re: Newbie Prop and WOT question

Welcome to iboats

You have a list of possible problems-

Fuel is old
Is there waterlogged flotation? < This will destroy performance
Is the engine running properly? Compression?
Is the trim circuit working properly?

You need to do a whole bunch of investigating before you spend money on another prop.

 

[Philster]

Re: Newbie Prop and WOT question

I've been told that I should run a 17" pitch to get 4600 rpm WOT by my local boat shop.

I hope what they meant was that this is how the rig should perform if all was in order, and weren't recommending a 17p prop.

Fact is, there is one or more of the following issues:

>The boat is waterlogged/rotting. The odds suggest this be investigated. Could be carrying around a heavy load of waterlogged foam and wood.

>The engine isn't putting out it's original HP.

You are down in pitch to a 15 (which represents a bandaid to recover RPM but lose top speed regardless) and it still can't get out of its way. I would think you have a weak engine plus a waterlogged boat. It's probably both issues combined.

 

[svkb]

Re: Newbie Prop and WOT question

I hope what they meant was that this is how the rig should perform if all was in order, and weren't recommending a 17p prop.

Fact is, there is one or more of the following issues:

>The boat is waterlogged/rotting. The odds suggest this be investigated. Could be carrying around a heavy load of waterlogged foam and wood.

>The engine isn't putting out it's original HP.

You are down in pitch to a 15 (which represents a bandaid to recover RPM but lose top speed regardless) and it still can't get out of its way. I would think you have a weak engine plus a waterlogged boat. It's probably both issues combined.

You think a weak engine with only 160 hours on it?

 

[svkb]

Re: Newbie Prop and WOT question

The engine RPM seems low . . . you need to factor in the gear ratio of the outdrive when calculating how fast a certain pitch prop should make the boat go (less prop slippage, of course) . So, it is hard to say . . . do you have any history on the boat as to what speed and RPM it has achieved in the past?

No I do not, the guy we purchased it from is a heavy duty mechanic (not that that means anything) and he just said the last time he had it out, it ran good.

In regards to the gear ratio, I believe the outdrive is stamped with a 2.0, but this doesn't make sense given the speed I'm attaining and the rpm. I'm wondering if the gear ratio has been changed.

A friend of the family is a very good boat mechanic so I'm going to get him to give it a look over.

 

[svkb]

Re: Newbie Prop and WOT question

I hope what they meant was that this is how the rig should perform if all was in order, and weren't recommending a 17p prop.

Fact is, there is one or more of the following issues:

>The boat is waterlogged/rotting. The odds suggest this be investigated. Could be carrying around a heavy load of waterlogged foam and wood.

>The engine isn't putting out it's original HP.

You are down in pitch to a 15 (which represents a bandaid to recover RPM but lose top speed regardless) and it still can't get out of its way. I would think you have a weak engine plus a waterlogged boat. It's probably both issues combined.

I was hoping a good tuneup, perhaps a new fuel filter would help.

How long does it take to get waterlogged? I don't think this boat has been in the water all that much. (boats in my area come out of the water at the end of the day) It has been stored indoors for a large part of its life.

Not disagreeing with you, just trying to understand how it could have happened.

 

[H20Rat]

Re: Newbie Prop and WOT question

How long does it take to get waterlogged? I don't think this boat has been in the water all that much. (boats in my area come out of the water at the end of the day) It has been stored indoors for a large part of its life.
.

Amount of time on the water doesn't matter, you could have a waterlogged boat that never left the trailer. The most common reason is someone storing the boat outside, with the plug in &/or the bow low so it won't drain.

 

[1FASTLASER]

Re: Newbie Prop and WOT question

is the current prop an aluminum or stainless? Also will the motor just not turn anymore rpms? If I read correctly this is what you are stating.

 

[81 Checkmate]

Re: Newbie Prop and WOT question

Just thinking out loud........ To check for water logged boat would be!
1. Weigh the boat and do a goggle search and find out how much it weigh'd when new... Compair notes.
2. After you pull the boat outa the water and get it home... is it still dripping water outa the plug hole 2 days later?

Also 2.0 on the drive sounds right....

If you are not water logged... then i would do a full tune up.... Change the fuel/water seporator if you have one.
Then give it another try and see what happens.

I take it you do not have a tach?

 

[svkb]

Re: Newbie Prop and WOT question

Just thinking out loud........ To check for water logged boat would be!
1. Weigh the boat and do a goggle search and find out how much it weigh'd when new... Compair notes.
2. After you pull the boat outa the water and get it home... is it still dripping water outa the plug hole 2 days later?

Also 2.0 on the drive sounds right....

If you are not water logged... then i would do a full tune up.... Change the fuel/water seporator if you have one.
Then give it another try and see what happens.

I take it you do not have a tach?

I have not taken the plug out because each time I check the bilge it is completely bone dry, even right after I take it out of the water. Not sure how I can weigh it as the trailer would factor into that weight and I have no idea what the trailer would weigh. I don't have a marina I can park the boat at while I weigh the trailer. Dry weight is supposed to be 2,200 lbs according to Searay specs. Any idea what the trailer would weigh? The trailer is the one it came with from Searay.

I pulled the plugs this spring and they all looked good, but thats as far as I went because I knew that the fuel was REALLY old and until I burned through that I didn't want to spend time and money on a tuneup. There should be almost none of the old fuel left now. I guess I should change that fuel filter.

Yes the boat has a working tach and I compared it to my timing light. The boat tach reads 100 - 150 rpm low. So my WOT confirmed with a timing light is 3900 - 3950 rpm and top speed of 33 -34 mph confirmed by gps. This was my initial question, with a 2.0 drive and (supposedly) a 15" pitch prop, how could I possibly be getting that kind of speed at that rpm? Unless the prop is not really a 15" (even though it's stamped as such) or the gear ratio has changed. I'm going to check the gear ratio tonight hopefully.

 

[svkb]

Re: Newbie Prop and WOT question

is the current prop an aluminum or stainless? Also will the motor just not turn anymore rpms? If I read correctly this is what you are stating.

It is an aluminum which needs replacing since I dinged it up on the weekend! :facepalm: It will not turn more rpms under load, I have not tried rpm that high on just motor, I really don't like revving motors that aren't under load, but I suppose I could try it for a second or two.

 

[svkb]

Re: Newbie Prop and WOT question

The previous owner stored it inside, but I'm not sure of the owner before that. I'm the 3rd owner I believe.

 

[svkb]

Re: Newbie Prop and WOT question

Just thinking out loud........ To check for water logged boat would be!
1. Weigh the boat and do a goggle search and find out how much it weigh'd when new... Compair notes.
2. After you pull the boat outa the water and get it home... is it still dripping water outa the plug hole 2 days later?

Also 2.0 on the drive sounds right....

If you are not water logged... then i would do a full tune up.... Change the fuel/water seporator if you have one.
Then give it another try and see what happens.

I take it you do not have a tach?

I will prop the front of the boat up tonight and see if I get any water coming out.

 

[svkb]

Re: Newbie Prop and WOT question

Thanks for all the quick replies already, you guys are super helpful!!

 

[1FASTLASER]

Re: Newbie Prop and WOT question

It is VERY possible that the prop is no longer a 15p prop. I have seen thousands of so called "repaired" aluminum props that are NO where near there so called pitch number. At this point I would obtain a borrowed KNOWN good prop and rerun your tests. Right now your numbers just aren't adding up correctly

 

[svkb]

Re: Newbie Prop and WOT question

It is VERY possible that the prop is no longer a 15p prop. I have seen thousands of so called "repaired" aluminum props that are NO where near there so called pitch number. At this point I would obtain a borrowed KNOWN good prop and rerun your tests. Right now your numbers just aren't adding up correctly

I was wondering that myself, given the fact that I live in the middle of the prairies, we do not have a lot of resources for good boat repair! I questioned the repair last fall when it was done, but it "looked" good.

It would also be virtually impossible to borrow a prop around here, but I'll definitely ask around.

 

[81 Checkmate]

Re: Newbie Prop and WOT question

Ok can't weigh the trailer seperated from the boat. If you can weigh the boat and trailer together and see what ya got.... Just for peace and mind tho...

I take it you store indoors always? This is just me but when i pull my boat i always pull the drain plug to see how much water i am taking on...people getting in and out of the boat wet,splashing and so on... the water has to go somewere, Plus i like to see if i have an unusal amount of water also...ie the bellows leaking or so on....

But i have to store outdoors so i always leave the plug out... rain water!

Others could be on to something with the prop refurbished.. do you have a local marina near by? Sometimes they will rent props for sea trial.

Any Boating friends with spare props?

Good Luck!

 

[H20Rat]

Re: Newbie Prop and WOT question

I have not taken the plug out because each time I check the bilge it is completely bone dry, even right after I take it out of the water.

I highly doubt you are waterlogged. A waterlogged boat will usually have a bilge that is a little damp all the time, if not wet. Like others mentioned, take the plug out some time and get the bow as high as you can. If you still see dripping 2 days later, probably water logged.

 

[svkb]

Re: Newbie Prop and WOT question

I highly doubt you are waterlogged. A waterlogged boat will usually have a bilge that is a little damp all the time, if not wet. Like others mentioned, take the plug out some time and get the bow as high as you can. If you still see dripping 2 days later, probably water logged.

The last time out (Sunday afternoon) I looked in the bilge and there was a tiny little puddle of water under the oil pan of the engine, I wasn't surprised because the water was pretty choppy and there was a fair bit of spray that made its way into the boat. I will pull the plug tonight and put the front end up as high as I can.