what do you think cruiz speed should be

[fkboatman]

1974 slickcraft 235, cuddy. has 188 mercruiser w/1.5 ratio drive with a 2bbl. its a 302 ford motor.boat holds almost 60 gallons of gas, and carrys a 8 hp kicker. take off boat gets on plain with ease, pulls very strong right to max rpm and i mean strong. for a large tub like this. motor turns 4 grand wide open in nauteral trim on drive. boat has no trim tabs. problem i have is that at cruz at about 3 grand its only running 23-25 mph but wide open with out trimming drive it will top at 41.7 on gps. just does not seem to have the mph to support the ease of getting to wide open. prop is a 14.5x17 vortech, iknow i could go up in a pitch to a 19 but then i will not stay a max rpm wide open,WHAT YOU GUYS THINKING.

 

[Texasmark]

Re: what do you think cruiz speed should be

The shallower the pitch the smoother the mid range cruising "sweet spot" will be. High pitch props make the boat tend to lurch i.e. one minute you are bogging down and you give it some throttle and the next minute you are going too fast sort of thing; you will get the max mph, but the quality of the ride elsewhere will be harder to control.

Some boats don't really seem to have a "plane out" where the boat comes out of the water, bow up, as it speeds up the bow drops the wake falls farther back down the boat, speed picks up at a faster rate and all. Many performance curves show that the best speed is usually just on plane. The boat gets up out of the water reducing drag and the engine is in a fuel comfortable spot on consumption.

If you have a hard time telling where you get on plane, look at the transom as you are coming out of the hole. When the water ceases to pile up behind the transom and smooths out where the only water there is coming directly out from under it, you are there.

HTH,

Mark

 

[JoLin]

Re: what do you think cruiz speed should be

Hi. You're topping out at 40+ mph, which IMO is very good for that size boat with a small block V-8. At 3000 rpm you're running 23-25 mph, which to me seems a bit fast for that RPM. 4000 RPM at full throttle seems low. Overall, I think you're overpropped (too much pitch).

What does Merc specify as the acceptable max RPM range for that motor? Generally, you want to prop the boat so it reaches the manufacturer's recommended maximum at full throttle. The rest falls into place once you've done that.

My .02

 

[tpenfield]

Re: what do you think cruiz speed should be

1974 slickcraft 235, cuddy. has 188 mercruiser w/1.5 ratio drive with a 2bbl. its a 302 ford motor.boat holds almost 60 gallons of gas, and carrys a 8 hp kicker. take off boat gets on plain with ease, pulls very strong right to max rpm and i mean strong. for a large tub like this. motor turns 4 grand wide open in nauteral trim on drive. boat has no trim tabs. problem i have is that at cruz at about 3 grand its only running 23-25 mph but wide open with out trimming drive it will top at 41.7 on gps. just does not seem to have the mph to support the ease of getting to wide open. prop is a 14.5x17 vortech, iknow i could go up in a pitch to a 19 but then i will not stay a max rpm wide open,WHAT YOU GUYS THINKING.

Yes, the numbers do not seem right. So, I did some prop slippage calculations for your cruise and WOT figures. At cruise you are getting 25% slip, which seems a bit high, but maybe typical for that boat. At WOT the figure comes out to only 3% slip, which seems unrealistic.

So, my thought is that your tachometer is not reading correctly at the high RPM range. You should see if you can connect an external tachometer (like what you would have with a timing light kit) and verify the RPM readings.

 

[fkboatman]

Re: what do you think cruiz speed should be

gps does not lie, boat is on full plane at 2700 rpms and does not need to go over 3 grand to put it on plane. boat has all new gauges and i checked tach against my snap on timing light with tach in it. its just about spot on. max rpm on that motor according to merc. is 4400 rpms. the motor turns 4 grand wide open with out trimming drive out. so i am sure if trim drive will max it out rpm at manufactures rpm max.

 

[steelespike]

Re: what do you think cruiz speed should be

I checked slip as well around 22-28% at 3000 and 22-25 mph.Reasonable percentage for that speed/rpm/boat size.
I'm pretty sure that motor is noted for its relatively low wot rpm.If accurate your wot slip is amazing.
It seems to me that your setup is really working well.If you trim up you'll likely get both more rpm and speed.
If you get your speed well trimmed out we can tell better if an increase in pitch is in order and what size.
An increase in pitch will usually slow hole shot, may make slow planing speed more difficult to control.
It appears based on your statement and specs that your propped perfectly allready.
A 19 will lower your rpm about 300-400 at 3700 your speed would be about 42+@ 5%slip based on your original 4000 rpm indicated speed.By the way best cruise speed is trial and error could be anywhere from just on plane to 600 to 800 higher a flow meter is likely necessary to tell any improvement
especially with your present performance.