Keith and Deborah... I am in the process of repowering my boat and have chosen the Perkins 4.108
as well. We have different sized boats but similar gear
ratios and prop sizes. I have done a considerable amount of research
into this engine's capabilities before making the purchase
. I also used David Geer's book and it did underscore the calculations I made elsewhere.
For what it is worth, I think the speeds you are attaining are consistent with the engine's output with respect to your boat's displacement
, LWL, gear
In performing calculations to determine whether the engine
is appropriate for my hull
and my cruising goals for my own repower
, I made extensive use of the power required calculator and the prop calculator from boatdiesel.com. This calculator is known to be right on the money
with it accuracy; and the results have been underscored with David Geer's formulas. I ran several calculations using the data you provided and have posted the results here. They are consistent with the results you are currently getting. The only thing I am noticing is that your prop seems overpitched. I ran the calculations for cruising at 2500 RPM
and performed separate calculations for 14, 15, 16, and 17 inch props. I also performed a power required calculation, which gives power ratings needed for each type of engine. The Perkins 4.108
is considered a "Pleasure" engine.
Hope this helps...