Possible cause of speed issues?

[KnotConnected]

Good morning!

I've had a few threads about my Cabrio 280's speed issues. This morning, the thought popped into my head: When we went out the other day it was mighty windy (in a direction that pushed me away from the dock and into the boat next to me at the marina, of course). So, I took down all the canvas but the front "windshield" panels, and it helped maintain control a ton. It was also apparent while trying to put them back on in the wind how sail-like the canvas pieces are.

In your healthy and respected opinions, could the three forward pieces of canvas in the picture below be causing enough wind resistance to slow me down 5 to 10 MPH?

http://i46.photobucket.com/albums/f117/n915aa/fixedstripes_zpsf5v7iybp.jpg

 

[Chris1956]

The rate at which a boat drifts is highly dependent on its exposure to the wind. However, a 5+ MPH drift is a whole lot for wind alone, due to the drag of the water. I think you have the right idea, but the wrong magnitude. Of course, if the wind and current are in the same direction, they can be additive.

 

[Alumarine]

More frontal area will reduce speed and increase fuel consumption.
I don't know how much though.

 

[KnotConnected]

For clarification, I'm referring to my top speed at WOT and cruising speed. Running WOT @ 4200 RPM I'm topping out at about 28 MPH, trimmed clean and on flat water. For long cruises up the bay I throttle back to ~3800 RPM and get about 18-20MPH out of her, making for a long trip. From what people with identical boats and power plants tell me, they're able to cruise at ~30-32 and top near 40.

 

[keith2k455]

Il throw my opinion as yes, I think it could be causing the speed loss. Try an experiment to prove it,leave it off for a while and see what happens

 

[shrew]

Is 4200 your max RPM at Wide Open Throttle (WOT)? Mercruiser 7.4 WOT is between 4400-4800. A Max RPM of 4200 would be indicative that you are overpropped.

 

[RGrew176]

Yes, 4200 RPM is below what that engine and drive can deliver. While not the same boat as yours I once owned a 1996 Bayliner 2855 Ciera. It too had the 7.4L engine and Bravo III drive. My engine maxed out at 4800 to 4900 RPM and topped out at 51 MPH according to my GPS. It cruised consistently at about 32 MPH at 3100 to 3200 RPM. You should be getting more out of that engine/drive combo.

 

[Bondo]

In your healthy and respected opinions, could the three forward pieces of canvas in the picture below be causing enough wind resistance to slow me down 5 to 10 MPH?

Ayuh,.... I really doubt yer canvas is the problem,....

But, I can guarantee the answer is in Here,....

 

[QC]

Rgrew mentions a 7.4L, and the OP says "identical ... power plants". I always caution against assumptions that they are all the same rating. Just a reminder to make sure you are truly comparing apples to apples. A 7.4L, 310? 300? 454, 350? 385? All 7.4 Liters. and would all perform differently. Other than that, I agree with Bondo, not the canvas. Simple way to eliminate that concern, test her down wind

 

[KnotConnected]

Thanks Guys! I'm not 100% on the 4200 RPM WOT. I thought that was what the manual said was in-spec, and I seem to recall last testing this that the RPMs were in-spec, but I'll check again.

 

[mr 88]

Is this a GPS speed your reading or the speedo on your dash that is using water pressure to move it. If its the dash speedo I would use your cell or some one elses and get a correct speed reading along with a accurate tach reading,Dash speedos or often way off the mark, Fish finders with the impellar/water wheel are fairly accurate.

 

[RGrew176]

Rgrew mentions a 7.4L, and the OP says "identical ... power plants". I always caution against assumptions that they are all the same rating. Just a reminder to make sure you are truly comparing apples to apples. A 7.4L, 310? 300? 454, 350? 385? All 7.4 Liters. and would all perform differently. Other than that, I agree with Bondo, not the canvas. Simple way to eliminate that concern, test her down wind

You are right. There should be more clarity. My 7.4L was a 300 HP carbureted 454 mated to a Mercruiser Bravo III drivel While comparing a 28 foot Bayliner to a 28 foot Larson is not an apples to apples comparison the performance numbers of my 2855 should be in the ballpark.