Want to go faster

[robgunther87]

Hey all, I recently bought a 14' Bayliner Mosquito hooked up to a 1981 Chrysler 85 hp outboard. Boat gets on plane pretty quick but tops out at 25 mph...which seems extremely slow for an 85 hp motor. Prop appears to be aluminum, and 19 is stamped on the side. I'm assuming that pitch degrees? I was playing around with engine trim last night and could only get 25 mph with the trim set all the way up. I'm still pretty green with boats/props, could anyone give me any advice on where to go from here?

 

[Frank Acampora]

Don't we all want to go faster?

Part of the problem depends upon the design of the hull and the year of the engine. A few photos would help. You can not just slap more horsepower on a hull and expect it to become a rocket. Understand that your hull is way overpowered and carrying a lot of weight on the transom. This puts a lot of the hull in contact with the water at plane and slows everything down.

I know what I am talking about: I have a 14 foot flat bottom hull (which is my test bed). I have had various engines on it from a 60 to a 125 and have gotten speeds anywhere from 27 to 55 MPH An old Chrysler 85 yielded 27 MPH. A newer Chrysler 60 gave 45. A newer yet Force 90 gave 50 and a 1990s Force 125 topped out at 55. In your quest to gain more speed, you absolutely MUST have a tachometer. You need to gauge RPM versus speed.

Depending upon the health of the engine, you MAY be able to substitute a 21 pitch prop or even a 23 pitch. However, the 90 used a 21 pitch at 5500 RPM and the 125 used a 21 pitch at 5900 RPM. Before you go monkeying about with props, gauge the health of the transom and reinforce it if necessary. Remember: You already have over twice the horsepower the hull was rated to carry. Think safety over speed at all times.

 

[Frank Acampora]

No Title

Here is my 15 footer boat with a 120.

 

[Frank Acampora]

No Title

Here is the 14 footer with a 125 on it. Note all the aluminum bracing on the transom. Note that both these boats use stainless props.

 

[robgunther87]

No Title

Don't we all want to go faster?

Part of the problem depends upon the design of the hull and the year of the engine. A few photos would help. You can not just slap more horsepower on a hull and expect it to become a rocket. Understand that your hull is way overpowered and carrying a lot of weight on the transom. This puts a lot of the hull in contact with the water at plane and slows everything down.

I know what I am talking about: I have a 14 foot flat bottom hull (which is my test bed). I have had various engines on it from a 60 to a 125 and have gotten speeds anywhere from 27 to 55 MPH An old Chrysler 85 yielded 27 MPH. A newer Chrysler 60 gave 45. A newer yet Force 90 gave 50 and a 1990s Force 125 topped out at 55. In your quest to gain more speed, you absolutely MUST have a tachometer. You need to gauge RPM versus speed.

Depending upon the health of the engine, you MAY be able to substitute a 21 pitch prop or even a 23 pitch. However, the 90 used a 21 pitch at 5500 RPM and the 125 used a 21 pitch at 5900 RPM. Before you go monkeying about with props, gauge the health of the transom and reinforce it if necessary. Remember: You already have over twice the horsepower the hull was rated to carry. Think safety over speed at all times.

Thanks Frank.

I researched the engine ratings for the Mosquitoes before I bought this one. 85 hp is the highest rated outboard for this boat. The boat was not equipped with a tachometer, and if possible I'd rather not install one. The engine runs very well and has no issues. With the current prop the boat gets out of the hole very quickly. I just think that 25 mph is pretty slow for such a large engine hooked up to a small speedboat. I think I'm going to go up in pitch like you suggested. Here is a pic of the boat.

 

[Frank Acampora]

Hmmm! I never saw a Bayliner so small rated for such high horsepower. It must be a specialty hull. Are you in USA? Is the hull an 81 year? Is it a "classic" that you do not want to change by adding a tachometer?

At any rate, Here are a couple of things you can do to try to get up around 45 MPH.. #!. You really MUST have a tach. At the very least, if you do not want to drill the dashboard, buy a tiny tach and mount it with double sided tape or Velcro.. 2. Even though relatively short, these engines do perform well with the anti-ventilation plate one inch above the hull bottom, whether it be flat or vee.. 3. They do best with the stock Chrysler props and stainless will deliver about 3 MPH more than stock aluminum. 4. If the engine does have power trim, then after the hole shot, USUALLY they like to run trimmed all the way out--however, hull design can influence that. 5. Be certain that the engine is set correctly. Watch the video on synchronizing carbs and timing. If you will consistently use automobile pump gas you can set timing to 32 degrees BTDC but be aware that 30 is safer. 6. Check the lower unit ratio. These engines will deliver way more speed using a 2 to 1 ratio lower. (turn the flywheel twice with the engine in gear. If the prop turns more than one turn it is a 1.76 ratio.

The engine does like to run at close to its rated RPM range of 5500 RPM. Prop it to achieve as close to 5500 as it can get at WOT.

Of course, check compression and timing to be certain the engine is internally healthy.

 

[steelespike]

Prop pitch is in inches prop size is written diameter X pitch. Theoretically a 19" prop will move 19" forward in one revolution.
But the prop slips in the liquid . This is measured in percentage of slip. Typical runabouts slip is about 12%.
The tach is very important for the health of your motor.
Boat motors work very hard. Imagine your car pulling a trailer up a never ending hill.
Your prop selection is like trying to pick a single gear for your car to get up the hill and do everything else.
As you go higher in pitch its like getting closer to over drive.Possibly lugging the motor forcing it out of its wot rpm range.
This can result in premature engine failure.As you go to a lower number prop it gets easier for the motor but there is the
danger of over revving. While your combination isn't likely to lug it is very likely to over rev.Just as destructive as lugging and more spectacular
with interior parts coming outside.
Speed without the rpm means nothing ,no way to determine accurately your speed and rpm relationship if the present prop works and
what changes may be needed.

 

[Sea Rider]

I you don't have a well trained ear to tach rpm engine by hearing them roar at wot, you need to install a tach, otherwise you're lost in middle of the ocean. As said, lugging or over revving engine is bad bussiness and could end in a costly one.

Happy Boating

 

[ron nh]

Here is the 14 footer with a 125 on it. Note all the aluminum bracing on the transom. Note that both these boats use stainless props.

WOW, does the motor out weigh the boat.

 

[Frank Acampora]

No, even with power tilt/trim that engine only weighs about 275 pounds. The 90 that it is rated to carry weighs about 250. 25 pounds does not make that big a difference and besides, I carry the gas tanks under the seats, well forward in the hull. The 15 foot (actually 15'9") Glastron V153 weighs 700 pounds bare.

The boat tops out at 45 with the rebuilt 90 Chrysler. It only tops out at 45 with the 125 but the engine is bone stock and really only produces about 90 HP. I prefer the 125 because of the better hole shot with the extra cylinder, and, of course, the mermaid on the cover.

The other boat is of unknown origin and the 140 Chrysler weighs the same as the 125--275 pounds. The original 1960s type flat bottom hull was probably rated to carry a 40 HP engine. However, she is highly modified to carry the engine on it now. I can get 55 and am trying to modify to get up to 60. I burn about 15 gallons an hour, run it for an hour, then take it home and glue it for a couple of hours. It is a fun ride when no traffic is around and I usually take it up the river during the week. Yes there are advantages to being old and retired.

 

[robgunther87]

Hmmm! I never saw a Bayliner so small rated for such high horsepower. It must be a specialty hull. Are you in USA? Is the hull an 81 year? Is it a "classic" that you do not want to change by adding a tachometer?

At any rate, Here are a couple of things you can do to try to get up around 45 MPH.. #!. You really MUST have a tach. At the very least, if you do not want to drill the dashboard, buy a tiny tach and mount it with double sided tape or Velcro.. 2. Even though relatively short, these engines do perform well with the anti-ventilation plate one inch above the hull bottom, whether it be flat or vee.. 3. They do best with the stock Chrysler props and stainless will deliver about 3 MPH more than stock aluminum. 4. If the engine does have power trim, then after the hole shot, USUALLY they like to run trimmed all the way out--however, hull design can influence that. 5. Be certain that the engine is set correctly. Watch the video on synchronizing carbs and timing. If you will consistently use automobile pump gas you can set timing to 32 degrees BTDC but be aware that 30 is safer. 6. Check the lower unit ratio. These engines will deliver way more speed using a 2 to 1 ratio lower. (turn the flywheel twice with the engine in gear. If the prop turns more than one turn it is a 1.76 ratio.

The engine does like to run at close to its rated RPM range of 5500 RPM. Prop it to achieve as close to 5500 as it can get at WOT.

Of course, check compression and timing to be certain the engine is internally healthy.

Hey Frank. Sorry I didn't respond sooner, I don't check this as much as I should. I am from the US, the hull is a 1979, Chrysler is an 81. I have not seen anything like it around, I figured that is because of it's age. Shes an oldy but a goody.... I'll start looking around for a small tach I could attach under the windshield. I simply don't have the room in the gauge cluster behind the wheel to make it fit. I was playing around with the trim the other night when the water was calm. With the trim set all the way up (it is a manual trim) I could squeak 28 mph. With the trim all the way down it was more like 23. I'll invest in a tachometer and go from there. Thanks for your input. Cheers!