Boat Speed?

[penrith]

Hi,

So I recently purchased an 1993 16' sylvan super select. It came with a 2 stroke yamaha 40 hp. I took her out with a couple of friends ( 3 people total in the boat ) and noticed that the max speed was ~ 11 mph. I expected to go ~ 20 mph with the 40hp. This is my 2nd boat but before I had a smaller utility boat.

I also had a 8 HP honda 4 stroke and that allowed me to go 5 mph.

I tested the cylinder compression / spark plugs for each cylinder. Everything checks out.
I didn't hear any inconsistencies with the engine when I ran it.
Can it really be a spun prop? Carbs need to be cleaned out?

I wanted to get some suggestions before I take it into the shop.


Thanks,
p

 

[Silvertip]

How did you check the speed. At 11 MPH you weren't even on plane. Speed should be closer to 30 MPH. Does this boat have a tach? If so, what was the reading at wide open throttle? Are you certain the engine is firing on all cylinders? You said you checked compression. What were the numbers. You said the engine ran ok. At 11 MPH it very likely was not running ok and quite possibly running with one dead cylinder. You've given us absolutely nothing to go on hence the questions. A spun prop hub would have the engine very likely over reving while you go nowhere. Carbs may or may not be the issue but we cannot see or hear your engine and your description is of little help. You really need to get a reliable speed readiing. Use the GPS feature on your cell phone. Boat speedometers are notoriously inaccurate.

 

[82rude]

A 95 year superselect is rated at 115hp.I would say your way under powered to begin with,but you should be able to get mid 20,s at least.All Silvertips questions are very valid.

 

[Silvertip]

With a max HP rating of 115, the rule of thumb for "reasonable" performance (75% of max) would mean you need at least 85 HP. Since your 40 is less than half of that, the boat is extremely under powered and 11 mph is about all you should expect, assuming everything else is in order.

 

[legalfee]

I have a 1995 Sylvan 824 Special Edition with a 90HP Force. I only get 18mph with two people on board and WOT at 5500 RPM. My pontoons need cleaning and that would help.

 

[Old Ironmaker]

So many questions on this end to give you any sound advice.3 people on board? Offensive linemen or Gymnasts. That could be a 1000 pound difference right there alone. Regardless only 11 mph there is something seriously wrong. With the info you have provided for someone to trouble shoot it online would have to be a mind reader or really really good.

 

[Old Ironmaker]

I have a 1995 Sylvan 824 Special Edition with a 90HP Force. I only get 18mph with two people on board and WOT at 5500 RPM. My pontoons need cleaning and that would help.

Force O/B is the problem right there. "L" drive? Tongue in cheek, sort of. Notorious engines over the years especially those year models Strap on a Merc, and hold on. For "Merc" insert the name of any other O/B. .

 

[roscoe]

Force O/B is the problem right there. "L" drive? Tongue in cheek, sort of. Notorious engines over the years especially those year models Strap on a Merc, and hold on. For "Merc" insert the name of any other O/B. .

Ummm, that would be a 24' pontoon.
Nothing to do with the Sylvan in question.

 

[penrith]

I really appreciate all the responses. Here is some more info,
1994 yamaha tlrs 40hp
cylinders 1 & 2 = 119. 3 = 116.
I also tested if each cylinder was firing by only having the spark plug connectors connected to 1 spark plug at a time and fired it up and she started each time. All 3 sounded very consistent. No gargling or anything.

I noticed that at full throttle it doesnt sound as high pitched as when i am at neutral and pull the throttle lever all the way up. ( is that ok to do )?

Ive been using my fishfinder which has gps, but I do not know if it actually uses the gps to determine the speed. Ill use my phone next time.
3 men avg weight ~180lbs?
I also had a 8hp honda kicker. Which kinda drags in the water a little b/c i dont have a mount for it yet.

I just ordered a plane fin for my outboard to see if itll help me plane.
Do they actually help a lot? Also the bigger the fin the better?

I hope 11mph is not my full speed. I'm planning to out on the water again saturday to try other things. Any more information would be great appreciated

Thanks,
-p

 

[roscoe]

The 16 Suoer Select is a 16' hull with a 79" beam and weighs 740#.
It is different from the Sport Select, and the Pro Select.
I can't recall the exact hp rating, but it is not 115.
I have fished in one several times and it moved quite well with a Mariner 50 hp, about 30 mph.


Penrith, if you have spark, and good compression, make sure you are getting fuel to each cylinder.

Does your boat have a tach ?
If so, what were your rpm's at 11 mph, wide open throttle ?

You should be hitting mid 20's when alone in the boat for sure.

 

[roscoe]

No, don't rev engine past 2000 rpm in neutral.

A whale tail fin may help, but they don't perform miracles.

You have to solve the problems first, get engine set up correct first.
Then, maybe try the fin.
Many people swear by them.
Many people swear about them.
They seem to work best on smaller boats and motors.

I'm guessing the the seller did not let you water test the boat before purchasing it.
That is always a good idea, then if the boat does not perform as desired, you don't buy it.

If possible, get us a photo or 2.
On the trailer, with engine vertical, side view which shows us the bottom of the hull and the engines lower unit.
A photo that shows this view:

 

[ahicks]

A patch at best. At worst, a very easy way to break the plate they bolt to right off. That plate is not designed for all the extra stress you're going to put on it with something like you're talking about. They break all the time!

"I just ordered a plane fin for my outboard to see if itll help me plane.
Do they actually help a lot? Also the bigger the fin the better?"

 

[wahlejim]

Lets start with an accurate tach reading and prop info. Those two bits of info will tell a nice chunk of the story.

 

[legalfee]

Force O/B is the problem right there. "L" drive? Tongue in cheek, sort of. Notorious engines over the years especially those year models Strap on a Merc, and hold on. For "Merc" insert the name of any other O/B. .

For sure. When I repower I'm looking at a Mercury 60 four stroke.

 

[Old Ironmaker]

Ummm, that would be a 24' pontoon.
Nothing to do with the Sylvan in question.

Yep, answering a thread within a thread, not good forumanship.

 

[Watermann]

Take it out lightly loaded and see what it does, the motor could be it's over propped for a boat that size or it's dropping a cylinder. Post the current prop dimensions.