Two Bayliner 288 boats, same engine, same bravo 3 but different WOT-gctid803158

[Guest]

I am still trying to nail down a performance issue on my boat, I have now finally found another person in the UK with the same Bayliner 288 and same Mercruiser 4.2 250HP D-Tronic diesel engine.

In a nutshell he cruises about 18knots on his and he has a top WOT of 28 knots with full tanks and a load out, when i bought mine originally it would top out at 22 knots.

So i started to do some digging and found out the ratio on the leg had been changed from a 2.00 to 2,20 but not the props so it had robbed it of 20% of its top speed. I went from 22p 3blade duo bravo props experimenting to 25p as I had a set spare set and that gave me another 2 knots, so I am now up to 24knots, but still 4knots short It might not seem much but WOT is currently very close to cruise speed so i need to get the maximum i can.

I have done the obvious, this season i sanded the hull and a fresh coat of ablative paint, the load out is absolute minimum, no water and half a tank of fuel.

His boat gets to WOT with ease while mine seems to be struggling, i have to drive it with the trim tabs burried all the time, if i raise them it will go off plane, basically she wont plane without trim tabs which cant be right, even at WOT.

Anyone else got any suggestions?

[2850Bounty]

"mctaggart" post=803158 wrote:
I am still trying to nail down a performance issue on my boat, I have now finally found another person in the UK with the same Bayliner 288 and same Mercruiser 4.2 250HP D-Tronic diesel engine............................................ .........................Anyone else got any suggestions?

[color]#000088 wrote:
Yes, trade boats with him while he is not looking!

Seriously...... if the only difference is the lower reduction 2.20:1 drive (that you have), and if your are Hell Bent on maximizing your performance, you may want to try a 2:1 reduction unit.

Have you been side-by-side of this other boat during a test run?

Have you verified your tachometer and speedometer?

In other words...... is his boat actually and truly performing better than yours?

[/color]

[Guest]

I have been doing the speed via GPS so it should be accurate, its not a huge problem, but its just bugging me knowing me that the boat can perform better with the same setup. I just want to rule out any issues with the engine.

I just had the props polished and balanced too so they are as good as they can get and the hull is clean as a whistle!

I am going to have a play around with the trim on the leg when i am out, i tend to run with it fully down, so maybe i can get a bit more performance if i tweek the angle on the power trim.

[2850Bounty]

"mctaggart" post=803164 wrote:
.....................................

I am going to have a play around with the trim on the leg when i am out, i tend to run with it fully down, so maybe i can get a bit more performance if i tweek the angle on the power trim.

[color]#000088 wrote:
Yes, as your speed increases and the hull comes up on step, by all means you will want to trim OUT some.

However, keep in mind that no matter how hard we try, we cannot prop ourselves out of an incorrect drive ratio! [/color].

[Guest]

a quote from http://www.go-fast.com/sterndrive_gear_ratios.htm

In my experience, a boat will run the fastest with a higher numerical drive ratio assuming you can find a prop with enough pitch to limit the engine to the desired RPM at full throttle. This does not take into account any handling or bow/stern lift issues.

The theory is that the faster a prop rotates the more power it takes just to turn it in the water. A slower turning prop with more pitch is more efficient than a faster turning prop with less pitch even though the calculation for theoretical speed is the same for both.

The chart on the left shows the prop shaft speed for a given engine rpm/drive ratio. You can use it to see the effect of changing drive ratio, without changing the propeller pitch. The difference between a 1.36 drive and a 1.5 drive is about 10%, or 500 RPM at 5000. If your boat was running 5000 RPM with a 1.5 drive the prop shaft RPM would be 3333. If you were to change to a 1.36, find the same (closest) prop shaft RPM in the 1.36 column and you see that the engine RPM would drop to 4500. As each inch of prop pitch is about a 200 RPM change, you would have to go down 2 1/2" in pitch to pull the same RPM. Our prop calculator will help with this math.

The overall ratio is a combination of both the upper and lower ratios combined. In the case of a 1.5 Bravo the upper is 32/27 (1.185) times the lower at 19/15 (1.266) = 1.50.

The downside to a higher reduction in the upper is that there is more torque on the lower gears, vertical shaft and prop shaft. In the case of a Bravo there is also more load on the gear floor.

Mercury has fixed many of the weak areas in the Bravo drive to varying extents with the new regular and X series.

[TenMile]

McTaggart -- when you do take your boat to WOT are you able to achieve the maximum rated RPM's for your engine?

[Guest]

In one of your posts you say your 4knots off this other boats WOT and you stuggle to stay on the plane just a thought how much weight do you have in your anchor locker if he's got rope and you've got all chain basicly you've got an extra 250-300kg hanging off the front of your boat. 4knots is not much to lose your engine might have more hours and be slightly tried lose a couple of hp bit of extra weight up front in storage lockers dosent take much for the wife to add an extra 100kg of stuff that "we might need" then there's the possibility of water sitting in your hull that you don't know about you add a few of these combinations together and your boat is way different to the the other one.