New Boat Feels Slow- Prop?

[Skiffhunter]

Hi,
First post, been reading alot trying to get some info. I have a new (to me) 15 ft Hobie Power Skiff with a 50 HP, 4 cyl, 2 stroke Mariner outboard. Research tells me, I should be getting 35mph WOT with a light load.

When I ran her, the GPS max speed was 25 mph, RPM max was 5800. It pulls hard, then seems to top out quick. The prop has a serial number and the last number is 13P. It has a Dolefin on the motor which appears to line up with the keel (+/- an inch or so). Not sure if I need more pitch or adjust the tilt on the outboard. Maybe take off the dolefin? It looks like there is some excessive spray from the dolefin when WOT. Could it be planing too high in back? I need more top speed. Please help. Thanks!

 

[junior1113]

Re: New Boat Feels Slow- Prop?

raise motor up on transom one set of holes at a time. all that spraying is uneeded drag which is slowing the boat down. u can try without fins. fins only work if motor height and tilt is correct.

 

[JimMH]

Re: New Boat Feels Slow- Prop?

There should be two numbers on the prop hub the first number is your diameter of the prop and the second is the pitch. You could try a higher pitch but I wouldn't go up more than on inch at a time.

 

[Texasmark]

Re: New Boat Feels Slow- Prop?

I'd jack the engine or just take the fin off till I got the problem solved. That engine runs 5000-5500 at WOT and has a 1.64:1 gearbox. With those high speed gears, it says that the 13P is probably correct as is probably the 35 mph at least.

Thinking........hmmmmmmmmmmm.

There is something wrong with your numbers or you have a spun prop hub...besides any problems you are (may/might be) having with the fin.

Look at it another way. The boat is used, yes? It had been operated by the previous owner for some time in the current configuration, yes? Therefore the setup must be good and it has developed a malfunction. A spun prop hub is my guess your malfunction per the numbers you issued (excepting your performance comment).

But a spun hub wouldn't give you a fast hole shot (your comment "it pulls hard"....yes?); would be just the opposite, but it could definitely give you the 5800/25 mph "tops out fast" (which I take to mean rpms max out fast) part.

So, I think you have an instrument telling you a story. Now the question, which one; speedometer or tach.....or we need to readdress the data.

Mark

 

[Skiffhunter]

Re: New Boat Feels Slow- Prop?

I'd jack the engine or just take the fin off till I got the problem solved. That engine runs 5000-5500 at WOT and has a 1.64:1 gearbox. With those high speed gears, it says that the 13P is probably correct as is probably the 35 mph at least.

Thinking........hmmmmmmmmmmm.

There is something wrong with your numbers or you have a spun prop hub...besides any problems you are (may/might be) having with the fin.

Look at it another way. The boat is used, yes? It had been operated by the previous owner for some time in the current configuration, yes? Therefore the setup must be good and it has developed a malfunction. A spun prop hub is my guess your malfunction per the numbers you issued (excepting your performance comment).

But a spun hub wouldn't give you a fast hole shot (your comment "it pulls hard"....yes?); would be just the opposite, but it could definitely give you the 5800/25 mph "tops out fast" (which I take to mean rpms max out fast) part.

So, I think you have an instrument telling you a story. Now the question, which one; speedometer or tach.....or we need to readdress the data.

Mark

The motor cant go up any further due to the way the transom holes and clamps are. Its a notched transom and if I move it up, the clamps will be above the notch.

The motor definitely pulls hard, feels good when you hit the throttle!- so not sure its a spun hub. Then it just peaks.. at 25mph (GPS). By "top out fast", I meant hits top speed. The tach shows 5800 rpm WOT.

The top of the dolefin is underwater at WOT, maybe causing drag? The boat is used and the older dude I got it from (original owner) said he put the dolefin on to help it plane better. He is a bigger dude and mentioned that he fished with a couple big guys as well. I will adjust trim then remove dolefin and adjust trim again. See if that helps, but also, it seems a longer pitch would help too.

Thanks for taking the time to respond !!

 

[Texasmark]

Re: New Boat Feels Slow- Prop?

Drag to me would load the prop and subsequently load the engine. You would be experiencing sub-standard rpm's not excessive rpms, unless the prop is cavitating which it could be doing as a result of disturbances caused by the tail being submerged.

Take the tail off and go get a set of data. We'll go from there.

Mark

 

[steelespike]

Re: New Boat Feels Slow- Prop?

Are there no holes in the mount itself to allow a higher position.Definately remove the fin.It could be causing it to lay down too much and is adding drag.Your hole shot will be slower without the fin.Your prop should be close to what you need.You
should varify your tach is accurate.You may be able to borrow a shop tach or buy a tiny tach; reasonable,easy to install and will fit any 2 stroke similar to yours.

 

[hwsiii]

Re: New Boat Feels Slow- Prop?

Hunter, just to backup Mark's statements to you, here is a chart and an analysis of what he is telling you and why he says that. With a 13 pitch prop at 5,800 RPM theoretically you could do 43 MPH, but you have a prop slip of 43% and that is ridiculous for that boat. Mark will get you fixed up and have your boat running right if you just listen to him.

H

 

[Skiffhunter]

Re: New Boat Feels Slow- Prop?

Thanks for the help guys. I will take her out this afternoon and post the results tonight. Lets get her dialed in!!

 

[Skiffhunter]

Re: New Boat Feels Slow- Prop?

Heres pics of how the motor is mounted to the transom

This pic is with the motor all the way down, dolefin on

Ok, so today I took the dolefin off. With the motor trimmed all the way down, no fin, I saw 28-29 mph and higher rpm (5900-6000). As I trimmed it out, speeds were similar and seemed to increase a touch (up to 29.8) as I went further out. I felt like the second or third hole down from all the way out was good. But the motor doesnt look inline with the keel in that position.Here it is trimmed out at the second hole where it was running ok.

Then I put the fin back on and trimmed it out to the same spots. RPM was back down to 5800 and speed was up around 27-29.

SO, trimmed out the fin works better then when trimmed in. When trimmed out, the fin is more visible at the surface while running on plane.

Maybe the motor is a touch low. What do you think from the pics? Just seems like a pain to lower it and I question the stability of the bolt through set up. I would have to drill my own new holes. See the pic, let me know what you think. I think a new prop with a 15-16" (?) pitch may be in order.

Its gettin there thanks to you guys!!(Im getting help from another forum as well)...keep the ideas coming. Thanks!!!

 

[trendsetter240]

Re: New Boat Feels Slow- Prop?

Hey I had that same motor over the summer. The gear ratio is 2:1 not 1.64:1.

That means your prop slip was actually 23% on your last test run without the fins. 5950RPM, 28.5MPH and 13P prop.

23% is still bad and you should definitely look at re propping. A 15P would be perfect if you keep all variables the same. i.e. same prop design and manufacturer, same material and same number of blades.

I'd look for a prop that will get you to 5500 RPM max as the recommended range is 5000 - 5500 as mentioned.

Now that the real gear ratio is known I'm sure the experts here can give you a more accurate recommendation.

Cheers.

 

[Skiffhunter]

Re: New Boat Feels Slow- Prop?

Hey I had that same motor over the summer. The gear ratio is 2:1 not 1.64:1.

That means your prop slip was actually 23% on your last test run without the fins. 5950RPM, 28.5MPH and 13P prop.

23% is still bad and you should definitely look at re propping. A 15P would be perfect if you keep all variables the same. i.e. same prop design and manufacturer, same material and same number of blades.

I'd look for a prop that will get you to 5500 RPM max as the recommended range is 5000 - 5500 as mentioned.

Now that the real gear ratio is known I'm sure the experts here can give you a more accurate recommendation.

Cheers.

Was yours the four cylinder? Any resource to find the ratio?
Anyway, gonna order a new prop and was considering a jack plate. Any suggestions on that idea? Or should I just redrill the transom..??

Thanks for the reply!

 

[trendsetter240]

Re: New Boat Feels Slow- Prop?

yeah mine was the 4 cylinder, 1985 50hp mariner. I have the service manual that's how I know the gear ratio. The 4cyl 50's were all 2:1 for that series, 1980 - 1985.

I don't think that engine height is your problem based upon your pictures. I would leave it where it is. Getting the right prop will make the most difference.

Your ideal setup would be 5500RPM WOT, slip of 10-15% and speed of up to 35MPH. That means a 15P prop.

Now the specific design of prop to get you there I don't know. Hopefully one of the previous posters, who are much more knowledgeable than I on these topics can make a suggestion.

 

[trendsetter240]

Re: New Boat Feels Slow- Prop?

Also if you resize your pictures to 640x480 they won't distort the layout so much

 

[Skiffhunter]

Re: New Boat Feels Slow- Prop?

yeah mine was the 4 cylinder, 1985 50hp mariner. I have the service manual that's how I know the gear ratio. The 4cyl 50's were all 2:1 for that series, 1980 - 1985.

I don't think that engine height is your problem based upon your pictures. I would leave it where it is. Getting the right prop will make the most difference.

Your ideal setup would be 5500RPM WOT, slip of 10-15% and speed of up to 35MPH. That means a 15P prop.

Now the specific design of prop to get you there I don't know. Hopefully one of the previous posters, who are much more knowledgeable than I on these topics can make a suggestion.

Also if you resize your pictures to 640x480 they won't distort the layout so much

Pictures sizes changed.

Heres two more shots

Definitely will get a new prop. You don't think the motor is too high? The dolefin is underwater when trimmmed down and I could see it skimming under the surface when trimmed back more. Maybe prop is keeping it from trimming up and out. But I'm still interested in hearing if the motor needs to move up.

Unfortunately, I gotta look at the tach now. I fished for a bit after testing and noticed the tach went dead. UGH!!!


Thanks for your input. All suggestions welcome

 

[Texasmark]

Re: New Boat Feels Slow- Prop?

Hey I had that same motor over the summer. The gear ratio is 2:1 not 1.64:1.

That means your prop slip was actually 23% on your last test run without the fins. 5950RPM, 28.5MPH and 13P prop.

23% is still bad and you should definitely look at re propping. A 15P would be perfect if you keep all variables the same. i.e. same prop design and manufacturer, same material and same number of blades.

I'd look for a prop that will get you to 5500 RPM max as the recommended range is 5000 - 5500 as mentioned.

Now that the real gear ratio is known I'm sure the experts here can give you a more accurate recommendation.

Cheers.

Thanks hwsiii for the support, but I am not the xpert here; just 50 years of experience.
-----------------------
The engine I looked up in the sales brochure and commented on was the later version 3 cyl 50, not the original 4 cyl design. Have no idea as to that gear ratio nor recommended WOT rpm range, and if you had one and especially with the manual who am I to argue.

So that puts a different slant on the prop pitch at 13" being about right and explains to me why the rpm's are so high and why you popped out of the hole and topped out so fast. A dinged up prop can add to cavitation which you are apparently still getting with the calculated 23% slip with your 2:1 gearbox. Since the engine is mounted so low and with the high deadrise at the transom, there is no other reason for the excessive slip....unless the hub in fact IS slipping, to me anyway.

In looking at where the engine is mounted on the transom and where the antivent plate is with respect to the bottom of the boat, it appears that the midsection on the engine is too long. The clamps are almost off the top of the transom notch and the antivent plate is still below the keel.

The other thing that appears to be supporting your problem is the lack of TTilt. It would certainly help you to get better high speed performance, tucking in to get a good hole and pushing out to get the boat out of the water at WOT.

Having your tach quit on you is not a good sign especially with your current problems. You have to know your correct WOT rpm's to accurately solve this problem.

Mark

 

[Skiffhunter]

Re: New Boat Feels Slow- Prop?

Thanks hwsiii for the support, but I am not the xpert here; just 50 years of experience.
-----------------------
The engine I looked up in the sales brochure and commented on was the later version 3 cyl 50, not the original 4 cyl design. Have no idea as to that gear ratio nor recommended WOT rpm range, and if you had one and especially with the manual who am I to argue.

So that puts a different slant on the prop pitch at 13" being about right and explains to me why the rpm's are so high and why you popped out of the hole and topped out so fast. A dinged up prop can add to cavitation which you are apparently still getting with the calculated 23% slip with your 2:1 gearbox. Since the engine is mounted so low and with the high deadrise at the transom, there is no other reason for the excessive slip....unless the hub in fact IS slipping, to me anyway.

In looking at where the engine is mounted on the transom and where the antivent plate is with respect to the bottom of the boat, it appears that the midsection on the engine is too long. The clamps are almost off the top of the transom notch and the antivent plate is still below the keel.

The other thing that appears to be supporting your problem is the lack of TTilt. It would certainly help you to get better high speed performance, tucking in to get a good hole and pushing out to get the boat out of the water at WOT.

Having your tach quit on you is not a good sign especially with your current problems. You have to know your correct WOT rpm's to accurately solve this problem.

Mark

Yea, need to get the tach fixed, hope its an easy fix... In the meantime, I think I'll order a 15" prop and hopefully see improvements, maybe the hub is slipping a bit? Then I'll look into a jackplate to move the motor up.

Is the antivent plate the same as the cavitation plate? Either way, seems like motor is too low.

 

[Texasmark]

Re: New Boat Feels Slow- Prop?

Yes on the name of the plate. I was raised that it was the anticav plate. But the guys in the know herein convinced me that anticav was a misnomer. The plate is to prevent the prop from sucking air from the surface which is ventilating. Cavitation is air associated with prop rotation regardless of the location to the surface.

Changing the prop solves 2 problems, too slight a pitch and a possible slipping hub.

The engine is still mighty low with respect to the bottom of the boat. Tweak hwsiii about that and the effect of the steep dead rise at the transom. He's good at that sort of thing.

The big guns on here say to fix your setup first then go after your prop, but I think in your case the prop definitely needs some fixin. Guess we'll see.

On engine position without trim: Your best overall operation would be with your engine vertical like in one of your first pictures. Tilting the lower unit to the rear like one of your latter pics, will help top end but can lead to ventilation and/or cavitation and extended hole shots. However, you don't know how bad till you try it.

I hope your new prop is cupped. Give hwsiii a PM.

Mark

 

[mpsyamaha]

Re: New Boat Feels Slow- Prop?

the motor looks really low to me. i would think a cheap manual jack plate would do wonders for this setup. with some experimenting you could find the happy medium of motor height and trim angle and see better performance than you have now. in your picture, the motor is really trimmed up high when in the second hole. i think if the engine was up higher (vertically) where it needs to be, you could see the higher rpms without as much trim angle. which should in turn cause less prop slip.

 

[trendsetter240]

Re: New Boat Feels Slow- Prop?

Pictures sizes changed.

Heres two more shots
http://i613.photobucket.com/albums/tt216/killerdnice/IMG_4350.jpg
http://i613.photobucket.com/albums/tt216/killerdnice/IMG_4352.jpg

Definitely will get a new prop. You don't think the motor is too high? The dolefin is underwater when trimmmed down and I could see it skimming under the surface when trimmed back more. Maybe prop is keeping it from trimming up and out. But I'm still interested in hearing if the motor needs to move up.

Unfortunately, I gotta look at the tach now. I fished for a bit after testing and noticed the tach went dead. UGH!!!


Thanks for your input. All suggestions welcome

From this angle yeah it looks to be too low. What's the measurement for your transom? That motor is already raised to it's highest without a jackplate. That makes me think that you have a 20" leg on a 15" transom.

You should measure the height of your transom at the centerline to be sure. It may be a better option to sell that motor and get something that suits the hull better.

my 2c.