alot of slip only at cruising????

[snowman51789]

I just got a new (to me) boat, its a 96 257 mirage trovare, it weights 4300lbs, it has a 454 mag MPI/EFI motor with 385 hp with a bravo 1 drive with 1.5 ration. the prop that I have on it is bravo 1 4 blade 22p.

I took it out and I wrote down my rpm vs speed

3400 RPM @ 37.3 MPH= 21% slip
3500 RPM @ 39.2 MPH= 19% slip
3600 RPM @ 41 MPH= 18% slip
4900 RPM @ 59 MPH= 13% slip WOT


my question is, why am I getting so much slip at cruising speed? my cruising speed is what I care about the most. would my cruising speed go up with a 3 blade prop? are these number bad and should I be looking for a better prop

 

[Sea Rider]

What's the min-max wot rpm range for that engine. Those posted results were while light, medium light or heavy loaded ?

Happy Boating

 

[snowman51789]

oops I thought I had everything, WOT RPM range is 4600-5000 rpm. those numbers were with about 7/8 of tank of gas, with 2 people and a cooler full of ice an beer. witch is how I loaded the boat is 90% of the time. (I always like to keep a full tank just in case I decide to go to a bar that's a cruise away)

 

[Sea Rider]

At 4900 rpm as usually loaded you're at near max wot range peak, why are you worried about more slip at 3400 rpm, do you often cruise at that rpm/speed ?

Happy Boating

 

[snowman51789]

I'm told that 3500 is a good cruising rmp (correct me if I'm wrong) so that is were I cruise at. At 3500 my slip is 19% from what I have read on here it seems like that is high. I was thinking it should be between 10%-15% (correct me if I'm wrong)

 

[tpenfield]

The fact that you are getting to the top end of your RPM range at WOT and a good boat speed to match would indicate that things are generally OK. Not sure if you had any current going with or against you during those measurements that might skew those slippage numbers a bit.

I did some prop slip calculations for my boat a while back and was a bit surprised to see numbers in the 20% range. See if you get substantially better numbers with 1/4 tank of fuel and 1-2 people and minimal gear.

Overall, I would not worry about it.

 

[Sea Rider]

Don't know if slip will lessen, one thing for sure is when fuel tank begins to empty, rpm will be increased accordingly throughout the rpm range. Good cruising rpm is relative to each combo, was it measured by fuel comsumption or enjoying trip while cruising ? what's good for others probably won't be same for you.

Usually throttle up to a rpm range that can hear engine running calm & smooth while maintaining good plane and that's at min 4700 out of max 5900 wot rpm as usually loaded. That's on flat calm no wind water cond, slight more rpm on slight choppy windy water cond to compensate.

Check at which rpm achieves what described. Enjoying is cruising at less or tad more rpm/speed, that's if in no hurry at all...

Happy Boating

 

[snowman51789]

So when people say that 10%-15% slip is ideal, are they talking about at WOT?

sea rider. Are you saying I could cruise at 4700 rpm?

What I mean my a good cruising rpm I mean im looking for the rpm that most people cruise at that won't be overly hard on the engine and drasticly decrease the life span of the motor. On my old boat (21 wellcraft cuddy with a 200hp 5.0 chevy) I cruised at 3200. This boat is on plane but not on plane very good at 3200.

I'm just worried about unknowingly binging overly hard on the engine.

 

[tpenfield]

If it makes you feel better, I found some prop slip graphs on the Web . . . this one is from the THT forum site.



There are others, but this is probably typical for many boats.You can see that the prop slip does not get below 20% until the higher end of the speed range (less hull in the water at that point). It is not likely that prop slip will be consistent throughout the speed range of the boat, due to the hydrodynamics involved on the hull.


So, your WOT rpm and speed along with the lower slip figure is indicative of all being well.

 

[Sea Rider]

Snowman,

Nope was a personal example of "best crusing" for my combo, engine revving smoothly and happy along excellent fuel comsumption. If your combo is somewhat hard to maintain on plane at 3200 rpm, 3500 or tad more will be ok while engine runs smooth to the ear and maintains good plane as currently loaded.

No issue for any engine running near or at max wot rpm of its range as usually loaded, once there can throtle at any rpm range of your preference...

Happy Boating