Engine-Propeller Matching for an Ambulance Boat

I've got a tricky situation on an ambulance boat that has a service life of approx. 15 years (10 m length, 3 m beam, 5-6 tons displacement, displacement hull, target maximum speed ~8 knots).

The challenge is, the end-user has already started procurement action for an engine and gearbox without a full matching study. I need to make this work, or understand the implications.

Here's what I'm working with:

• Proposed Engine: 100 HP at 2400 RPM
  
  
• Proposed Gearbox Ratio: 3.46:1
  
  
• Proposed Propeller: 3-blade, 330mm diameter (pitch not given)
  

My NavCad Findings:

Using a generic engine model in NavCad and assuming a propeller pitch of 230mm (since it wasn't provided), my initial analysis shows a mismatch. However, if the gear ratio is reduced, NavCad indicates the boat could achieve 6 - 7 knots, but without utilizing the engine's full power.

My Urgent Questions:

1. Propeller Pitch: What pitch range would you suggest I investigate further for a 330mm diameter prop on a vessel like this, especially given the existing engine and gear ratio?
   
   
2. Gear Ratio Implications: The previous propulsion configuration does not have a gearbox listed in their machinery (tis may be an oversight). But is a gear ratio of 3.46 is too high?
   
   
3. Shaft RPM Safety: What's a generally safe continuous shaft RPM range for an engine and driveline of this size (100HP, 2400 RPM engine max)? I'm concerned about avoiding critical vibrations or wear.
   
   
4. Any other concerns: Given these pre-selected components, what else should I be wary of or investigate?
   

Any insights or advice on how to best navigate this situation and optimize performance would be hugely appreciated!

 

There isn't enough information to give you a proper answer. For example, what is the maximum diameter propeller that can be installed? The shape of the hull will influence the drag, therefore the power required to move at 8 knots. A gear ration of 3.46:1 is very high, particularly for the planned power, which appears excessive. Unfortunately, some things can't be made to work properly.

 

The existing propeller (which is to be kept as it is) has a diameter of 330 mm. The hull shape is a round bottom. I thought the quoted gearbox ratio is too high too. I will try to post some of the results I got from NavCad so you'll have a better idea. Thank you for your insights.

 

A prop diameter of 330 mm (13") is complete nonsens for a 6 ton boat, you will waste 75 percent of input power at the target speed, even with the least bad gearing. With a wl length of ~9-ish meters on 6 tonnes, you have a slenderness ratio of ~5 and a Froude (volume-) number of 0,95. I'd be surprised if you got a total drag/lift ratio below 0,07 in those conditions. That requires a thrust of about 4200 N, which means a propeller loading in terms of kt/lambda^2 of 2,4, which is waaaayyy too much.

 

I agree; it won't work.