Hi
I have just run the BlueBoat with the standard motor mounts, and also with a 12degree adapter.
The adapter points the motors down at the rear, with the intention to reduce the pitching that develops when at ever increasing thrust/speeds.
It does make a difference!
Is there a reason not to do this?
See the attached photos, with the adapter, and without the adapter and the adapter as fitted.
Best regards Chris C
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Hi @ChrisCole, welcome to the forum! 
Certainly an interesting idea - I’ve passed it on internally to see if anyone has particular thoughts.
I imagine as shown:
- it would somewhat increase the drag of the motor within the water, because of the step, and
- there would be some reduction of forward thrust efficiency from the motor pointing less forward, and/or
- an increase of hull drag from more of the hull being in the water.
That said, pitching the motor does seem like one option for reducing vehicle pitch at high speed, especially in scenarios where payloads need to be horizontal but are unable to be pushed towards the front of the vehicle 
Hi @ChrisCole -
Very cool test! I think in calm water this works well, but in rougher conditions the extra lift at the stern will lead to a dramatic loss of efficiency, as the propellers, when closer to the surface, will more often “suck air.” Entraining air like this results in a dramatic loss of thrust… Have you seen this happen at all?
Hi I should perhaps try and mitigate my lack of experience! I started model boating in the '70’s, later went to sea as a ship’s engineer, 2nd’s ticket and part A Chief’s, then lots of other things…I hope not to offend, but offer my understandings not as any expertise but curiosity?
The subject certainly has many opportunities!
Regarding the pitching, this occurs at all speeds, just the rate varies.
The skin friction of the hull is just a function of the design. I guess we have to accept the levels of skin friction, due to the design of the hulls? We surely want a horizontal boat, as this would aid better survey work? If the boat is ballasted flat at rest, and then dynamically runs flat, should this not be an important consideration? For a hull running flat on the water, the biggest resistance to surface craft is not the skin friction, but the wave making resistance. Hence bulbous bows etc.. Also submarines go so much faster underwater, as though the skin friction area increases, the wave making is zero, and by far makes up for the extra skin friction.
The adapter as tested was a quick lashup, and certainly adding a flared blade between the vertical face and the hull would be a great improvement. Needs to take account of the water cooling slots. (I don’t have much success getting fillets and chamfers to work in Freecad.)
Of course having the props mounted up the stern post/transom, does reduce the prop tip to water level clearance, but the alternatives would be to have smaller faster props, lower down, or even under the keel. The best compromise is where they are now.
Would we really be considering surveying in stormy weather? Certainly if our props are coming out of the water survival becomes more a concern than completing the survey.
- There seems to be a factor that influences the pitching, and that relates to the thrust line passing below the centre of buoyancy. The greater the vertical distance then the worse the pitching. A fast short boat would seem to amplify this effect.
- Also there has been work carried out which claims that at inclination angles below 15deg, the forward thrust actually could increase, due it is surmised to the lower blade approaching the water flow faster, and increasing it’s thrust while the upper blade retreats from the water flow, and reduces thrust to a smaller scale.
- Is our aim to run the boats flat, beneficial to surveying etc., or is pitching acceptable? There is not so much a lifting of the stern, but a keeping the bow down effect. Accepted we have a turning couple, but could it not be argued that the boat is not so much pivoting round the centre of buoyancy but lifting up and forward, along the thrust line, maintaining a flatter attitude? This is a dynamic action, not just static.
When I first started model boating I was advised by an old hand, that if I maintained a prop shaft angle of 12 to 14 degrees, then my boats would always run flat. And it has worked! I don’t think the BlueBoat was intended to go up on the plain?!
Note air bubbles and cavitation bubbles are not the same thing!
Centre of gravity is static, but the centre of buoyancy moves!
Is the above a load of nonsence, or even relevant?
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In my experience operating the blueboat in really poor conditions, the pitching up attitude is a good thing that prevents it from submarining through waves. I’d be concerned with a flat attitude in those conditions (not that there is any indication of water ingress when plowing through waves).
I’m not sure who you’re quoting there, but both statements are at least generally correct (and potentially tangentially relevant).
Typical air bubbles can occur when air is pulled into the water from the surface (e.g. by a propeller blade scooping air and pulling it into the water as it rotates down), or when gas is released into the water by some other means (e.g. exhalation of a diver, implosion of an enclosure, or geysers/vents at the floor of the water body.
In contrast, cavitation is caused by a rapid reduction in pressure (e.g. at the low pressure side of a fast-moving propeller blade) that dips below the vapour pressure of the fluid, which causes a bubble of gas (e.g. steam) to form before rapidly collapsing, with a (potentially damaging) impact as the bubble collapses. It has no requirement for gas to be introduced from elsewhere (as the bubbles are created from evaporation of the fluid), so can be harder to avoid than external air sources.
Thruster performance is significantly reliant on the density of the fluid being pushed by the propellers, so any presence of gas bubbles in the water is unwelcome. That said, externally introduced air bubbles tend to be more impactful on thrust performance, because a lot of gas can be pulled in or released at once, whereas cavitation is generally more locally confined, but causes surface damage (pitting/erosion) and is a significant source of noise.
The centre of gravity of an object is determined by its mass distribution, which usually is a static attribute of a vessel, at least in operation. There can be changes if a vehicle is equiped with a retracting/movable arm, or a variable ballast system (e.g. some boats take on and release water to adjust stability in different operating conditions), and there are often changes between operations if payloads or ballast weights are introduced, removed, or simply moved around.
Buoyancy force is determined by the submerged volume, so the centre location is typically static for underwater vessels (subject to different pressures and fluid densities), but is quite variable for boats because there is only partial submersion, and vehicle pitch and roll change which region is submerged.
Gravity, buoyancy, drag, and lift are all forces a vehicle is subject to, and depending on their relative strengths and how and where they are applied, they can either cancel each other out, or contribute to substantial components of the vehicle’s motion (e.g. a glider is reliant on lift and drag to adjust its pitch and roll, and potentially its altitude, while for an ROV with vectorised thrusters, external forces are often undesirable from the perspective of straightforward control)
I was sort of quoting from college lecturers but as said that was a long time ago, and the memory is not always too sharp.
Many thanks for the clarifications.
I am still enjoying the BlueBoat and it’s capabilites. I have learnt to live round the magnetic interferance from the metal lined pond. So long as I give it plenty of time and keep away from the pond edge, I only get a momentry xy error as I launch.
Still looking forward to the RTK GPS!
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