FSH 210 Sport
Jetboaters Fleet Admiral
- Messages
- 7,362
- Reaction score
- 9,160
- Points
- 512
- Location
- Tranquility Base
- Boat Make
- Yamaha
- Year
- 2020
- Boat Model
- FSH Sport
- Boat Length
- 21
I’ve used this online calculator for many years, mainly for jetting purposes. density altitude calculator When I was flying I used the flight slide rule.
The rule of thumb for horsepower loss in normally aspirated engines is 3% per 1000‘ of elevation gain. However, that assumes standard atmospheric conditions at the higher altitude. For example, at 5000’ the rule of thumb dictates 15% power loss, however if density altitude is added to the mix on a summer day, the horsepower loss can easily be 27%-30%.
When you add the horsepower loss for high altitude AND being over propped, its a double whammy as now the engine cannot spin up to the rated rpm so the power loss is even greater. This is where a centrifugal driven type of supercharged engine can see a lot of benefit as well since the super charger boosted air is directly proportional to the engine speed.
Another thing to consider is engine power / prop-impeller power required curve. In most situations there is always reserve power to drive the impeller so peak rpms can be achieved. In the case of my boat when all those factors are considered I’m down about 38% on power as my engines will only rev up to 6500rpm wIth full fuel and 6 people on a hot day. As mentioned above it is common for a boat owner to re prop their engines so that the engine can run in the peak power range.
Installing properly pitched impellors on your boat is the single biggest performance enhancement you can make.
The rule of thumb for horsepower loss in normally aspirated engines is 3% per 1000‘ of elevation gain. However, that assumes standard atmospheric conditions at the higher altitude. For example, at 5000’ the rule of thumb dictates 15% power loss, however if density altitude is added to the mix on a summer day, the horsepower loss can easily be 27%-30%.
When you add the horsepower loss for high altitude AND being over propped, its a double whammy as now the engine cannot spin up to the rated rpm so the power loss is even greater. This is where a centrifugal driven type of supercharged engine can see a lot of benefit as well since the super charger boosted air is directly proportional to the engine speed.
Another thing to consider is engine power / prop-impeller power required curve. In most situations there is always reserve power to drive the impeller so peak rpms can be achieved. In the case of my boat when all those factors are considered I’m down about 38% on power as my engines will only rev up to 6500rpm wIth full fuel and 6 people on a hot day. As mentioned above it is common for a boat owner to re prop their engines so that the engine can run in the peak power range.
Installing properly pitched impellors on your boat is the single biggest performance enhancement you can make.
