34 Convertible cruise speed

[TonyD]

We have a 340 with twin Yanmar 300 HP. We're only getting about 14-15 knot cruise and 18 knot top speed. Does anyone know if that's all there is with these engines or do we look at the props??

Thanks for any info.

Tony D.

[PaulR]

We have the 34 convertable (2000) and it has 315 yanmars. We are seeing a cruise around 20 knots with top end 24-25 knots. We are going to get some new props and hope to increase the speed by about 1-1.5 knots.

Check out your factory specs for engin output rpm's and see if you are running low or high of that when you are on the pins (the 315's are rated 3800rpms). That will give you a good indication if you need some new props.

[frederick]

We have the 34 convertable (2000) and it has 315 yanmars. We are seeing a cruise around 20 knots with top end 24-25 knots. We are going to get some new props and hope to increase the speed by about 1-1.5 knots.

Check out your factory specs for engin output rpm's and see if you are running low or high of that when you are on the pins (the 315's are rated 3800rpms). That will give you a good indication if you need some new props.
Paul R...did you get new props? If so can you tell us the new size they are and who much they improved speed etc...thanks

[The Good Life]

TonyD

Those engines are capable of performing beter than thatb but there are many factors that could be affecting there performance. The bottom of the boat should be clean and the props must be match to your boat. I have a pair on a 1992 32' Convertible that I have put 450 hours of service and I may be able to help you with your speed problem.
When you say that you get 14knots at cruise. What are your rpms when you cruise at 14 knots? and what are your rpms when it reaches the top speed of 20 knots?
You most also verify the accuracy of your tachometers by using a phototach. If they are not reading correctly it is difficult to go to the next level in the quest of tunning your props to your boat.
It is also important to know the size and pitch of your props and the reduction gear ratio of your transmitions.
For instance if your tranies are 2:1 you can use bigger props that if they are 1.5:1.
I know that your boat should weight arround 17,000 without fuel and water. How much fuel did you have in the tank when you could only reach 14 knots at cruise? and how many people did you have on board?