Significant Drop Off in Top Speed

[knotflying]

Hmm, perhaps I need to contact a Volvo tech too - but it costs almost $200 in transit time for them to even come over. Am I the only R-31 that can't get past 19kts? At my original sea trial the Volvo tech thought either my engine "might" be not making enough power or there might be too much pitch in my prop since we couldn't spin quite the RPMs he was expecting.

Corey

Wouldn't your issue be covered under warranty if it is an engine issue?

 

[coreychamness]

Yes I assume it would be covered if indeed an issue with the Volvo. Unfortunately just to get the Volvo tech to my marina is expensive just for his transit to drive over. Nearly a $200 investment to find out. It is just under 200 nm for me to make the voyage over near Annapolis, MD. It is probably the only way I will find out answers though this thread shows even that takes persistence.

 

[ttlmac]

Corey, I have a 2014 R31 Sedan. On my first trip upon taking delivery last summer I hit 25 knots with a following sea, and I believe that when I had the engine certified and we tested max speed with and against the current, I averaged in the low 20's. I have added some weight to the boat since then with a hardtop and more "junk" on board, but it seems that the boat is running less effiently now than even earlier this summer, and the peak RPMs are lower. I've been following this discussion, but am not sure whether there really is any problem on my boat. I love the Volvo engine, it's very smooth and seems extremely rugged. However, the NMPGs are down a couple tenths from earlier in the season. So, I'm following this thread with interest, but am not yet concluding that there is anything wrong with the engine.
Tom
Indian Summer II

 

[CAPTCRUNCH]

Is it a coincidence that all/most of us posting about our underperforming boats/engines are in the Mid-Atlantic area?

 

[coreychamness]

I am dredging up an old posting from last year because I have a new data point. We could barely reach 18 knots most of the boating season last year even though we could originally go over 23. This week we hauled the boat out and had two coats of Hydrocoat SR applied and the hull washed and waxed. I didn't really have enough time for a speed trial but I took about 90 secs and pushed the throttle up enroute back to home port and was accelerating past 20 kts in short order. We had an ablative paint on the bottom and there did not appear to be all that much growth/slime but apparently it made a tremendous difference. I don't know if we can still pull 23.4.

Corey

 

[nzfisher]

First off I am not a mechanic but if the boat has been sitting around a while with out being used and you haven't put any toxins in the fuel to keep down the algae (or whatever it is that grows in diesel) I would look to the fuel first. It is also best to keep your tank(s) full to stop water condensation (and probably slow growth of algae). Fuel and batteries are the number one causes of agita for boaters. My guess is unlikely prop unless you nailed something when you were out and you would notice that.

 

[trailertrawlerkismet]

Corey:

Interestingly enough we had Kismet in Florida waters for three months and our hull speed dropped from 21 MPH to 18 MPH, from the beginning of the trip to the end. Shortly before we left I had a diver go under the boat and clean the hull. We took the boat out the day after the hull cleaning and the speed was back up to 21 MPH. You are correct in that it does not take much growth to slow a boat down. Those that boat full time in Florida often have there boat hulls cleaned on a regular basis because of the rapid algae and barnacle growth.

Jim F

 

[jroyle]

Any growth at all on our boat bottom drops our speed down 2-3knots..Its fun returning the boat to the slip after a new bottom paint, so fast..it really is amazing how the barnacles etc. will cut speed and increase fuel consumption.

 

[knotflying]

Also keep in mind that every little new thing you put on the boat adds to the weight and hence speed reduction. One will be amazed at how the things add up as you have the boat longer and longer, especially when you live on it 3 months at a clip.

 

[coreychamness]

Jim,
You are so right. All I really had on our bottom was some slime and a little algae. I have ablative paint though so I really can't scrub the bottom during the year cause it will just wear the paint away for little purpose. Last year I had a different paint so we'll see how Hydrocoat SR does.

 

[ttlmac]

This topic was covered at length late in the season last year with many people including me reporting a drop of in top speed and performance. I just put my boat back in the water with a newly painted bottom but otherwise no changes. The performance of my boat (2014 R31 sedan with 300hp Volvo) is now very close to when new. In my first checkout ride this weekend the engine RPM was back to just under 3500, and the max speed was in the low 20s, all with a full fuel tank and a couple passengers. So, my conclusion is that even a small amount of growth on the hull significantly impacts performance. I would guess that later in the season I will lose some speed and efficiency.
Tom
Indian Summer II

 

[coreychamness]

I am in agreement with Tom.

 

[Tardis]

Thanks Tom.
My max even after a diver cleaned the hull was 14 knots at 3050 rpm.
Just got the bottom painted, and now back to 22 knots at 3400 rpm.
Agree it does not take much growth to slow these down.

 

[Automaticman865]

Hi,
Been through this same problem with my Cutwater 28. Had the Volvo rep out and was on a freshly painted bottom only a week old and we were only able to reach 3300 rpms. To make a long story short we've changed out the prop from a 19x23 to a 19x21. What a difference it makes. We were able to hit the rev limiter same as the performance chart for a brand new CW 28. Thats around 3630, and fully loaded with 3 adults and full fuel and water and can still hit the redline of 3500. Volvo recommends that if rpms are down at full throttle I should still be no more that 100 rpms below redline. Speed does drop off a few knots but the important thing is the engine is running at the right rpms.
Stuart

 

[MartyC]

I had about the same situation with my 2016 C28 as Stuart did. When the boat was new and Volvo did the initial check out we only turned 3370 with 3 people and very little gear on board. On our last trip to NW, with the boat loaded for cruising, we were only turning about 3250 WOT. ACME suggested re pitching from 23 to 21 to get the rpms close to 3500. We sent our new spare prop to Performance Propeller in Soddy Daisy, TN. They did a great job. We now turn 3500-3540 WOT.

 

[stwendl]

Just a few points to consider. Engine performance as we see it in speed in the water is dependent on environmental conditions, like air temperature and pressure, water temperature and its density.

A boat in salt water will sit higher then in freshwater due to its density. That bit loss of floatation in freshwater adds up to more friction on the hull, more water to be used aside, and less thrust on the prop.

Air temperature and density has an effect on the engine performance.

Add this all up and some your problems may be explained beyond just a dirty bottom hull.

 

[baz]

Also, when testing for WOT (or any other speed for that matter) one should go in a straight line one way and then reverse and go back along the same line and then take the average of the two runs.

Wind direction, tide state, current direction and wave/swell directions can influence speed significantly. Also close all the doors, hatches and windows. 🙂 Also does the boat have the vertical drag inducing or sail affect/benefit of a dinghy on the stern ?